An ailment is an effect on a target that lasts for a duration. Most ailments deal damage, apply stat debuffs, or both, and most can be applied to the same target multiple times (or "stack").
Ailments can either be applied by a hit, or by area effects that attempt to apply ailments at a fixed interval.
Most ailment applications are chance based. However, having an ailment chance greater than 100% allows you to apply multiple stacks at once if that ailment can stack.
For example, if you have 235% ignite chance with Fireball, each Fireball hit will apply 2 stacks of ignite, with a 35% chance to apply a 3rd stack of ignite.
Each damaging ailment, such as Bleed or Ignite, has a fixed base damage, but is affected by damage modifiers from the skill tree and attribute scaling of the skill that applies it.
Even when an ailment is applied by a melee skill or a hit, that does not make it count as melee damage or hit damage and does not cause it to be affected by stats like increased melee damage.
If a target has the maximum number of stacks for a given ailment, attempting to apply another stack of that ailment will replace the oldest stack.
The most common ailments are described in other pages within this section. If an ailment isn't described here, the tooltip (or alt tooltip) of the node or item that grants it should explain the ailment's functionality.