Some features are global to all types of possession (although some are only available to xanafication specifically):
Tower Takeover: If a warrior is xanafied, they will be able to use Code XANA in any Towers to activate them in exchange for Energy (or for free if the tower is already activated by someone other than XANA)
Particles: Possessed warriors can be identified through colored particles representing the warrior's new master surrounding them.
Skin Change: Possessed warriors who have set a custom skin for xanafication will automatically swap to it. You can learn more in the Customization page.
Core Attack: Any xanafied warrior is capable of launching attacks on Lyoko's core in Sector Five and help Monsters damage it.
Riding Monsters: Possessed warriors can ride certain monsters such as Tarantulas, Krabs and Mantas if they are part of the same faction (e.g xanafied players can ride XANA's monsters.)
Frontier Loss: If a possessed warrior is sent to the Frontier, they will go back to normal.
Tower possession is the more relatively common type of possession.
They can happen at anytime whenever XANA launches a Possession Attack, targeting only warriors currently on Earth, or when a warrior launches a possession process in a tower to jeremiefy someone (which requires Knowledge.)
While on Earth, the possessed warriors will have unique abilities:
Lightning Bolt: A laser that will damage any enemies but can also be used to damage objects (e.g Supercomputer circuits or Scanner fuses.) The default key is F (off-hand switch) to attack and right-click to destroy objects (left-click for elevator panels).
Super Jump: As the name suggests, possessed warriors can jump very high to reach safe places or get to their goal faster. This can be used by jumping while crouched.
Super Speed: Possessed warriors are a lot faster than usual. This is a passive ability and does not need to be triggered.
Disguising: This ability allows possessed warriors to hide amongst the rest of the group. While disguised, all powers become unavailable. However, their skin will go back to normal and the particles will stop appearing, allowing the warriors to wait and strike at the right time. Disguises can be toggled by pressing F (off-hand switch) while crouching by default.
Once virtualized, all powers will become unavailable aside from the ability to use Code XANA.
Technically, any entities capable of activating towers is capable of possessing someone. This means possession by Hopper or Tyron is theoretically possible, although not available in normal gameplay.
If someone is possessed by Hopper, their chat messages will be reversed, just like Sissi's speech in the original show.