A 'zombie' (also known as zeds, walkers, roamers, lurkers, biters, Z's, Meat Bags, Freak Bags, and zak) is the term associated with a person infected by a virus; an infection that reposes the brain and shuts down the internal systems of the victim. After this mutation occurs the victim is no longer a person, but instead a mindless shambling corpse with an insatiable hunger. In contemporary versions these are generally reanimated or undead corpses. Stories of zombies are as old as the human race, with mentions of them in the oldest known work of literature, the Epic of Gilgamesh. But further more, zombies can be thought of an existence of life after death.
Other more macabre versions of zombies have become a staple of modern horror fiction, where they are brought back from the dead by supernatural or scientific means, and eat the flesh of the living. They have very limited intelligence, and may not be under anyone's direct control. This type of zombie, often referred to as a Romero zombie for the filmmaker that defined the concept, is archetypal in modern media and culture.
Most Zombies can be found roaming around their places of death in search of living organisms to feed on. The zombies will walk around, searching for food until it locates a living being at which point it will raise it's arms and form a guttural moaning deep in its throat, attracting other zombies in the area.
In a first world country (and many second/third world countries), most people showing symptoms of infection (fever, disorientation) will be brought into the closest hospital. After a few hours (depending on the severity of the infection) the infected will "die" and then reanimate, a fully developed zombie.
Prior to Skydark the US government was researching a virus quite similar to the zombie plague, at a redoubt in Kansas. It lay dormant for nearly 100 years until Lariat unleashed it upon the Deathlands as the rotties plague. (Wretched Earth)