The undead are, as you may imagine, humans who were once dead that have now returned to the land of the living. There are three kinds of undead.
Revenants are undead who are fully sapient. Essentially, mundane humans who died and are now human again. They are functionally immortal and do not need to eat normal food, sleep, breathe, etc. They were brought back by extremely powerful either fae or necromancy magic. They do tend to look a little pale, and their hearts do not beat, nor do their hair or nails grow. Their eyes can sometimes look a little fogged over as well. However, they do feel an urge to eat brains. Unlike Risen or Liches, who are being kept alive by continual Necromancy magic, Revenants were brought back once, and now require some sort of specific enzyme only found in brains. No one knows why. Without brains, Revenants will become full of rage and potentially go on rampage, killing humans until they can get their hands on brains. They can only be killed by completely destroying their bodies with fire, otherwise they will "live" forever, if you can really call it living. They are stronger than the average human. This is not contagious, they cannot turn other people into Revenants.
Risen (or Rotten)
The risen are undead who were raised by a Necromancer or a Lich for more nefarious purposes. They can be controlled in a very loose manner, like a puppet, or they can shamble around on their own. They don't have the capability of speech or sapient thought, and on their own will eventually fall dead again.
A Lich is a Necromancer who has traded their humanity for immortality, storing their soul in a safe talisman or artifact of some kind and their body becoming nothing but an empty shell. They look similar to a Revenant, but unlike a Revenant, they can come back after their body is destroyed by acquiring a new freshly deceased body. Since their soul is contained within an object anyway, they cannot die unless that object is destroyed.
Notable Character Canons
Artem Marcello is the mayor of Rookline and also a Revenant, unknown to anyone except his assistant. He was brought back after a car accident during his campaign that killed him & his younger brother, and he does not know how or why.