User can possess or take over the body of any living being, whether sentient or not, by stealing their motor functions and senses. Users may or may not have to leave their own body for this to take place. Disembodied users such as spirits may use the body as their own.
Temporarily/permanently inhabit the body of the target.
Take complete control of a target's body, even using it as your own (the target is then known as a vessel).
Control a person's actions.
The user's mind/spirit/soul can leave their body freely.
Skilled users can possess multiple host bodies while retaining their own or even while occupying another vessel.
When the user possesses a body, they gain the skills and powers of the body.
Can possess a body which can be useful for espionage and can lie dormant inside the person's mind and body.
Utilize as a last resort to escape a dying body and to achieve immortality.
A demonic hyena (Buffy the Vampire Slayer) possesses a group of students.
The Beast (Doctor Who) possessed Toby Zed so he could escape Krop Tor ...
... and possessed Sanctuary Base's Ood as well to act as his Legion.
An Eight leg (Doctor Who) possesses Sarah Jane.
The Mara (Doctor Who) has possessed Tegan on various occasions.
Louie Preston (The Haunted Hathaways)
Miles Preston (The Haunted Hathaways)
Luke and Emma (Jessie) being possessed by evil spirits.
Erick (Los Protegidos) can possess any body.
Androvax (Sarah Jane Adventures) possessing Sarah Jane Smith.
Countess Margaret Isobel Thoreaux (Smallville) possessing the body of her descendant Lana Lang.
Anubis (Stargate SG-1) has no body and uses a human-shaped force-field to keep his energy form intact. Without it, he can possess any human and use their body as his own, but the body will decay as a result of holding him.
The Leviathans (Supernatural) possessing Castiel and his vessel, Jimmy.
Lucifer (Supernatural) possessed Nick, his plan b vessel. But because Nick is not his true vessel, his body is decaying from Lucifer's sheer power.