Charm effects are a form of crowd control that gives control of a victim to the caster. This control can either be direct, giving the caster the same control as a Hunter pet, or his own character, or indirect, causing the victim to "switch sides" and physically attack his former allies with no additional actions needed from the caster. Most charm spells effectivly put the caster to sleep while he controls his target.
The Undead racial ability 
[Will of the Forsaken] breaks charms and renders a player immune to sleep, charm, and fear effects for 20 seconds. Charmed creatures will not be freed by damage, but damaging and "waking" the caster usually will.
Any charm, no matter the duration listed, will last a maximum of 15 seconds when used against players in PvP.
Examples of abilities that charm opponents:
- Priest:
[Mind Control] - Engineers: Gnomish Mind Control Cap, Gnomish Universal Remote (mechanical targets only)
- Warlock: Enslave Demon (demons only)
See also: Crowd Control, Root, Snare, Sleep, Stun, and Fear.