| A corpse raised to undeath by necromancy.
"It seemed that while the zombie came from the grave, it was neither a ghost, nor yet a person who had been raised like Lazarus from the dead. The zombie, they say, is a soulless human corpse, still dead, but taken from the grave and endowed by sorcery with a mechanical semblance of life-it is a dead body with is made to walk and act and move as if it was alive. People who have the power to do this go to a fresh grave, dig up the body before it has had time to rot, galvanize it into movement, and then make of it a servant or slave, occasionally for the commission of some crime, more often simply as a drudge around the habitation or the farm, setting it dull heavy tasks, and beating it like a dumb beast if it slackens."
Zombies are the reanimated corpses of once-living creatures and are a type of derived undead. They are considerably slower, stupider, and clumsier than whatever they were when they were alive and are unable to heal, but on the flip side, they do come with undead resistances to cold and negative energy.
Zombies may appear in the Dungeon as random monsters, or be created by players (or other monsters) through Necromancy or the Invocations of certain gods. Note that hostile zombies can serve as a "preview" of upcoming monsters: since they're weaker than the original, the zombie version of monsters will often show up a level or two before the live version.
As servants, zombies are extremely useful for characters who need meat shields, and make up the main component of a necromancer's or death knight's strategy. Often simply hiding behind your zombies and waiting is a good strategy in battle against strong foes; what works even better is reaching past your minions with polearms. Note that a newly-animated zombie can pick up weapons and armor from the square it was animated on (usually their old equipment), but that's your last chance to equip them, as they can't pick anything up or wield/wear new items. You also won't be able to recover equipment from your zombies until they are destroyed, so if you want, say, Psyche's dagger, make sure to take it off her corpse before raising it. Despite having significant disadvantages, zombies can still be very strong, particularly in the case of strong physical monsters or well-equipped uniques.
Zombies created through players' Necromancy spells are temporary and will eventually crumble to dust, but using Yredelemnul's Animate Dead ability creates zombies that persist until they are destroyed. Additionally, zombies are unable to use stairs, but you can lure monsters up or down stairs into the arms of your zombie horde.
Although zombie versions of each monster will have different stats, Crawl tends to lump them together, making identification at a glance difficult. In console mode, zombies will be separated into zs and Zs, representing large or small zombies. Tiles mode is more specific, providing unique tiles for several categories of monster, but there is still a lot of possible variation within those categories. In either mode, you can always press x and highlight the zombie in question to determine exactly what type of monster you are dealing with.
Below are all the categories of zombies in tiles mode: