Deadites are creatures, most commonly people, that have become possessed by evil spirits (demons) in the Evil Dead universe. However, monsters and creatures released by The Necronomicon Ex-Mortis can be referred to as Deadites as well.
Human beings that have been possessed by a Deadite will exhibit white, pupil-less eyes, dark grey skin, and will also appear to decay rapidly. In The Evil Dead & The Evil Dead 2: Dead By Dawn, when people became possessed, they continued to decay indefinitely. It can be assumed that eventually the host body will decay completely from the demon's corruption. In The Evil Dead, Professor Knowby warned via his tape recordings that he feared the only way to stop those possessed by the demons (Deadites) was throught the act of bodily dismemberment. Also in the same film, he warned that the book said that these enduring creatures may lie dormant but were never truly dead.
When a human becomes a Deadite, it is assumable that they have no control over their physical body. Their souls are apparently taken to Hell and tortured, although this might just be the demons' threat to scare those not possessed. In the films, no one who has ever become a deadite was ever released or returned to normal except in a few cases, the most notable being Ash or Mia from the remake.
The Deadites also apparently have complete recall of the lives of their vessels, their memories, their emotions, and their relationships, and are able to temporarily appear as fully human again to communicate with those not possessed. This has only been used to escape from certain death or indefinite confinement. Through this trickery, the Deadites provoke an irrational and emotional response in their captors or would-be killers to allow them an opportunity either for escape or to catch them off-guard.
The Deadites as creatures appear as beings of pure debauchery, lust, and insanity. They enjoy self-mutilation, swearing, and relishing the fear of their prey. In Evil Dead (2013), the Deadites inflict terrible bodily damage to themselves before they even begin attacking others. It is unknown if they do this as a scare tactic or simply because they enjoy the pain. Oddly enough, after being a Deadite for several hours, suffering various injuries (mostly self-inflicted) including forking her tongue with a box-cutter, and being buried alive, Mia was completely unharmed after release from her possesion. This may imply that while possesing a human any harm on the body is inflicted on the demon rather than the possesed.