Being a Zombie

Objective

Zombies win the round by killing all humans. When a human dies, they become a zombie. Zombies lose the round if any humans escape.

Starting Zombies

The humans furthest from a sigil, or closest to a zombie spawn if no sigils are present, will become zombies. The percentage of humans that become zombies varies by map, but the amount is usually around 11%. On maps with bots, you can expect a lot of those slots to already be taken by bots which always volunteer to be a zombie.

Redeeming

Once a player has become a zombie, they can only become a human again through redeeming. If a zombie kills enough humans, they will redeem and become a human themselves. A zombie will need 3 kills to redeem on waves 1-2 and 4 kills on waves 4-5.
Zombies cannot redeem while playing as a boss zombie. Kills acquired as a boss zombie will count towards the quota, but another kill as a non-boss zombie is needed to redeem.
Zombies cannot redeem on wave 6, or when disabled by an objective map.

Classes

Press F3 to bring up the class selection menu. Zombie classes are divided into three types: destruction, support, and assault.
Destruction-type zombies specialize in destroying barricades. Support-type zombies specialize in strengthening other zombies or weakening humans via repeated projectile attacks, and Assault-type zombies specialize in killing humans, usually those not inside a barricade.

Bosses and Demibosses

A boss zombie will spawn 5 seconds before the start of wave 2 and onward. Boss zombies have a high health pool, ranging anywhere from 2000-6000 HP, and various abilities exclusive to them.
Demiboss zombies are in between normal zombies and boss zombies. They unlock on wave 2. Most demiboss zombies are weaker versions of boss zombies. They have a moderate health pool, ranging anywhere from 1000-3000 HP.

Obtaining Boss and Demiboss

Zombies which perform well are awarded with boss or demiboss.
100% of human damage and 25% of barricade damage are used in determining who will be the next boss zombie. Zombies that have already played as a boss are less likely to be selected again as they require additional damage in order to outrank their competitors.
Demiboss is obtained by damaging barricades, humans, and sigils. When any of these entities are damaged, a demiboss meter will begin to fill up. Once this meter is full, a zombie can press F2 to become a demiboss. This will instantly kill the zombie, unless they happen to be a boss, and set their next spawn to whichever demiboss class they have selected. Pressing F2 again before the zombie respawns will stop them from spawning as a demiboss and retain their demiboss progress. The demiboss meter will fill up at faster or slower rates depending on whether zombies are currently losing or winning, respectively.

Nests

Nests are zombie spawnpoints that have been built by the zombie team. Zombies will automatically attempt to spawn at the nest closest to humans, but zombies can manually spawn at any nest by holding ALT and clicking on a name. Zombies can "lock" their spawn to a specific nest by holding ALT and clicking the lock icon next to a nest name. The flesh creeper/weaver, a normal zombie, can construct nests by holding RMB at any spot on the map. Bonemesh, a boss zombie, can throw nests on the ground by pressing RELOAD. Nests can be destroyed by pressing LMB on them as a flesh creeper/weaver. Humans can also destroy nests. Nests cannot be placed too close to humans or uncorrupted sigils and require enough space to spawn a zombie.

Brood Nests

Brood nests are a type of nest created by the Brood Queen. Unlike regular nests, brood nests may be placed on walls and ceilings. The only zombie which may spawn from a brood nest is a Broodling. A broodling is a small headcrab that does not collide with humans. Zombies will not spawn at brood nests by default, they must be manually spawned at by using the ALT menu.