WARNING: This article contains MAJOR SPOILERS relating to
some of the most pivotal parts of Worm. Although all articles may contain spoilers, this article will reveal major plot twists and should almost certainly be avoided unless you have finished the story.
"I'm guessing there's more that I don't know about?"
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Shards, also known as faeries, passengers or agents, are fragments of incomprehensibly immense colony organisms that serve as the source of parahuman powers.
Shards resemble crystalline structures folding and unfolding like hypercubes.
Each Entity is comprised of "trillions upon trillions upon trillions" of Shards. When two entities collide, rubbing against each other, their Shards can be damaged or destroyed by the friction and pressure, but they are exchanged between the two. Before coming to Earth, Eden and Scion cast off the vast majority of their Shards.
They travel along predetermined paths after being cast off by an Entity in space, "raining down" on the Earths over the course of decades. Once they landed, Shards resided in other worlds, uninhabited worlds, and they remained cloaked and concealed in areas humans were unlikely to explore. 
"All of our powers stem from the same source. It’s this big alien bastard that we keep seeing when we have our trigger events. Except each of his cells is coded with just a fragment of his brain and a technique he uses to manipulate his environment, protect himself or attack others. He spread powers around Earth as part of a way to stress test them. He wants to leverage our brains and imagination to figure out ways to make the most of these abilities or innovate new ones."
Many dead Shards were also damaged. A dead shard is not connected to the larger shard network and by extension the entities. 
The large numbers of Eden's Shards were damaged, corrupt, or dead. This meant there were more parahumans than the entities had intended, rendering them useless to the cycle. Vikare's Shard was such an example, though it was still able to grow properly as long as he was alive.
Scion would generally destroy them on sight as he saw them falling.
Some Shards were vital to an Entity, and not intended to be given out. If their vital processes were separated, they could die. Scion saw some of Eden's vital Shards seemingly falling and being sent to hosts after she crashed.
Sometimes, a Shard begins splitting off, ready to find a (generally young) host similar to the original target and attach to them with the benefit of more experience. This generally takes time and/or stress on the part of the host. Given enough information and lessons learned, it's possible for a Shard to bud multiple times. This split-off fragment will have a derivative ability. Ideally, the bud will exchange information with the parent Shard, but this requires the hosts to remain in proximity to each other. It's possible for a Shard to bud before or at the same time as a trigger event.
Very rarely, the host will experience a second trigger very similar to their first trigger, causing the bud to consolidate in the current host instead.
When Shards bud or cross-bud there can be interaction between the parent Shards creating interesting effects. For example Golem's power derives partially from his Shards relation to Kaiser's and Allfather's, the manifestation of his power as limbs and similar derives from his mother Heith however.
Powers from multitriggers are from a mix of Shards. The Shards give up a small to moderate portion of themselves, and then leave the rest of themselves to develop normally while taking in info from multiple sources. The Shard will usually connect another, primary host as well, possibly with some relation to one of the grab-bag capes. This is primarily intended for the end of the cycle, and serves to stress-test powers and compare and contrast the smaller powers.
Ever since they first evolved on their home world, Entities have been composed of "clusters" of Shards, each with different functions.
Different Shards tend have different abilities, even if the powers they grant humans appear similar. Generally they are not supposed to give a specific power; different hosts and trigger events produce different powers. Since it furthers the main purpose of Cycles, which is recombination and search for possible applications of specific shards.
Although there are some specialized "sleeper"-shards, which will wait until specific trigger-type conditions, that synchronize with their specialization better, and skip the host if there is a bigger potential for their favorite trigger situation to happen earlier. For instance, "Administrator" shard falls into this category according to Wildbow's example of alternative timeline.
Every parahuman receives only a small portion of the potential abilities of their Shard. For example, a Shard may choose to apply it's powers to only one type of target; insects, humans, trees, etc.
Shards also limit themselves to avoid damaging the host. "Thinker Headaches" are one example of this.
The effects that Shards have on their hosts are dependent on several factors, including but not limited to: age of the host, the host's personality, the host's trigger event, how much the host embraces the Shard's need for conflict, and the nature of the Shard itself.
Generally the younger a person is the more heavily their personality is influenced by their Shard. The connection becomes "broader", removing or exaggerating almost every element of their personality. This does not necessarily mean that they are more powerful.
Some parahumans have particularly deep connections with their Shard when their personality is naturally in alignment with the Shard's desire for conflict. The Shard does not influence the person's personality as heavily, but the line between the two nonetheless becomes blurred.
Parahumans with very little connection to their Shard generally find that either they have "episodes" where the Shard takes over, or their power systematically acts according to the Shard's desires rather than the host. Examples of this include Garotte, Echidna and Leet.
Bonesaw speculated that she and Jack Slash both had deep connections with their Shards. The Fallen are generally very good at feeling the connection with their Shards.
Shards draw on a number of different factors to identify their host, including DNA, electromagnetic patterns, and traits humans don't have names for.
Individuals with the same DNA as a parahuman, even a deceased one, would possess a Corona Pollentia right from the outset. They still required a trigger event to gain powers, but it was extremely easy.
↑Link two shards together for cross-purposes, to augment one power (ie. means of increasing range or penetration of organic matter added to a shard with blaster/striker type power) or to add another power to the package (ie. Tecton's structural awareness added to the tinker shard), and you get... a shard. Each shard is a collection of data and mechanisms, and each 'shard' as we think of it is already composed of multiple shards. - Wildbow on SpaceBattles
↑They can alter the shards they put out there, before the shards hae mature, because a connection is maintained. No connection = 'dead' shard. - Part of a Comment by Wildbow on spacebattles
↑dead damaged shards are disconnected from the greater entity network that allows living shards like Zion's to function. - Comment by wildbow on Reddit
↑ 7.07.1Generally the trigger cause fits in the same general category. Brian had his second trigger for much the same reason he had his first. They can differ in nature. What's happening is that the entity is drawing from context and exploring/conceptualizing new uses for the powers (which are still in the metaphorical computer's memory, but not in the hardware that burned out in the trigger process). The entity begins splitting off, ready to find a generally young & similar host to target (piggybacking off the parent's context & experience for an easier triggering process/analysis) but then a major event prompts it to catalyze and consolidate in the current host instead. If it's in an adult before finding its way to the child, it can begin this splitting-off process (generally requiring time or a degree of stress to allow for the maturation). They are exceedingly rare (two noted in-story. Taylor didn't second-trigger in the last arc, to be clear), and generally speaking they do more harm than good. If it's a straight power-up, you're probably doing it wrong. - on SpaceBattles
↑Q: Do Brandish and Lady Photon have closely related or bud shards?
A: Yes. Bud shards. One had it, then the stress they were under prompted it to bud, it found the nearby host who met the prerequisites. - Wildbow on SpaceBattles
↑1.) Powers from multitriggers are from a mix of shards. The shards, instead of going all-in, give up a small to moderate portion of themselves, and then leave the rest of themselves to develop normally while taking in info from multiple sources, just like a shard would naturally gather info and eventually reach the point where it could bud. Except in this case, the shard will usually just go find another host, maybe with relation to one of the multi's, maybe not, and empower them.
2.) There would need to be some common ground. It's very possible that, in a car accident where two individuals triggered, that one would get seriously hurt and be anxious about 'oh fuck, my life is ruined, what do I do!?", While the other is like "Oh fuck! I'm going to jail!" while being a little bruised. The former gets a major brute power with minor thinker, while the latter gets a minor brute power with a major thinker. And the shards want to keep them involved and interacting, so in pretty short order they start to really hate or really like each other.---Comment on Reddit
↑Multiple triggers are actually the endgame of the cycle, prior to the reabsorption and collapse. When virtually all individuals in the setting are parahumans, connected to shards, the introduction of multiple-triggers serves to stress-test powers and compare and contrast the smaller powers. - Dice Rulebook
↑Q: Are shards generally more likely to use more similar or more different mechanisms? For example, are two electricity controlling capes usually using the same method to control electricity, or different methods? Is someone like Panacea using a kind of precise telekinesis that could be used more generally and is used more generally in other shards, or is her shard using mechanisms that specifically work on biological materials?
A: Depends on the shards. Generally different. - Wildbow on SpaceBattles
↑If a shard can give any power classification, depending on the trigger, Wildbow:For the record, some shards can, but not all. Some shards are almost always going to just trigger Master, for example, and you just won't trigger unless you're in a state of mind that lends itself into a frequency match for the scan-manton snap (as Bonesaw describes in 11.h). - comment by WildBow on reddit
↑ 15.015.115.2When we get our powers, the passenger manages this sort of scan, trying to figure out a way to apply a part of itself. So Taylor gets a power that's restricted to bugs, Canary gets a power that's limited to people. At the same time, the passenger kind of figures out if there's any danger of the power harming us, physically or mentally, and it sets down safeguards and limits. Headaches like Dinah or I get are part of that. [...] Every power has secondary uses, uses that are locked away. - Cockroaches 28.4
↑Jack had a different kind of connection. A deep connection. He was in alignment with the particular nature of his passenger. The passengers naturally sought conflict, and Jack had fed that need from very early on, and he had sustained it for years. The line between the two was so thin as to be impossible to mark, but Jack's personality remained his own. Altered, but not subsumed. - Interlude 25
↑Some people have very little connection to their shards. Look at Leet - his shard actively sabotages him. Look at Echidna, or Sveta. You end up with disconnects, and stuff starts to stand out as the shard's behavior becomes more distinct, either in 'episodes' or a pattern of the power not cooperating. - Wildbow on SpaceBattles
↑the fallen hard to stamp out, unpredictable, and tend to live on the fringes of society, where they're harder to track and heroes need to devote far more effort to squirreling them out. There's also a tendency to give more power to the lunatics and assholes, because it furthers their nebulous agendas. They want to be loathed. In a more abstract sense, shards love conflict, and the fallen are very good at feeding it, so the fallen get rewarded by the shards. Breadth and depth. - Comment by Wildbow on Spacebattles
↑ 20.020.1Various elements that were unique to every individual served as a signal that the passenger could reach out to in an attempt at reconnecting with a host. DNA, electromagnetic patterns, patterns she could barely measure with instruments, all contributed, none was absolute. Once the connection was established, powers were possible as well. A moment of trauma sped the process along considerably. Her initial assumption had been that coming to life would be enough for the clones. The Corona Pollentia was developing as the originals did, drawing from DNA to form as a lobe in the brain, right from the outset. The dreams formed the connections between the corona and the clone. The bonds were forming too quickly and easily. - Interlude 25
↑The shard, being alive, reports back with Scion and the broader network, and then goes to find a new host.
- Wildbow on Spacebattles
↑Was there any of the original personality in there? The memories of the person that was? If there were, then it implied something ugly. Glaistig Uaine collected passengers, tapped them for power, and if this thing had memories, then what did that suggest about the passengers? - Excerpt from Extinction 27.4