A walker from The Walking Dead.
Undead is a state of being and refers to indivudials who are no longer clinically alive, and yet... somehow, they find themselves re-animated and may even possess human sentience. The undead are most commonly found in the horror genre and take the forms of vampires, ghouls and zombies. It could be argued that part of qualifies an individual as undead is the lack of a human soul, but as the definition of such a concept is somewhat nebulous, it cannot be ascertained with any degree of consistency. One program of the supernatural horror genre that addresses such an issue is Buffy the Vampire Slayer. The vampire characters on Buffy are actually a sub-sect of demon. When a person becomes a vampire, a demonic spirit takes control of their body, excising the human soul. The demon spirit absorbs the individual's knowledge, memories and personality, but are otherwise governed by infernal desires. These souless beings no longer possess the capacity for genuine human emotion such as love, compassion or sorrow. One of the major plot points addressed in the series is that a vampire can sometimes be "cursed" with a soul, at which point, they are usually overwhelmed with guilt over their past atrocities. Zombies have no souls. They just stink.
Another distinction between the undead and the living is that the physical vital signs which govern the functions of basic life are no longer in effect. Vampires and zombies do not have a respiratory system, so they do not breathe as human do. A vampire's circulatory system is geared towards the consumption of blood and does not operate the same as a human's. Brain functions appear to be the same between the two, but how such a thing is maintained is up to the interpretation of the storyteller. They do not poop.
On AMC's The Walking Dead, the area of Atlanta, Georgia is wiped out due to a zombie plague. The dead inexplicably return to life and begin chowing down on any slow-moving pedestrian they come across. Referred to as "walkers" in the series, these undead cretins are monsters in the truest sense, with no semblance of humanity whatsoever. Plus, they're really friggin' ugly.