Zombies are undead humans. They are usually slow and often have the ability to remain even when decapitated. The zombies in TimeSplitters: Future Perfect were actually created by Jacob Crow and The Brotherhood of Ultra Science in his attempt to become immortal. Zombies, such as the shock zombies (which could fire electricity) in particular were early 'prototypes' for the TimeSplitters army he would later go on to create.

The zombies hold the record for the longest inhabitence of time, they appear from as early as 1895 to 1994. Zombies can be created by various means, by attack from other zombies, election, magic and an aura which the TimeSplitters possess.
In the Mansion level of TS3 there is an opportunity to infect a scientist with the virus which will turn him into a zombie. You may also use this as an excuse to brutally murder the zombie in a variety of ways, depending on how cruel you are!

Varieties

Generally, in the games zombies are undead. Often times zombies are confused as characters who have risen back from the dead. There are a variety of zombies, usually depending on their location and way in which they were created.
In TS1 and 2 the zombies seem to be aware of their environment, as they chase the player (or hostage in some situations) without banging into walls or falling over (most of the time). Admittedly, in TS2 the zombies do not seem to be aware of stairs, and seem to fall down them all the time. It is also unknown how zombies in the dam are intelligent enough to enter and use the elevators (!).

In TS3 the zombies have been carefully modelled to react to their environment. In the mansion zombies frequently trip over crates and fall down staircases. There is one report that a zombie managed to detach its own head on a ceiling beam.

It is unknown how Crow manufactured the zombie's ability to wield a shotgun, or indeed run at a character with the sole intention of mauling him/her to death. As Jo-beth remarks, "Who hit the fast-forward on these guys?"