Snare effects are a form of crowd control that reduce a victim's movement speed, preventing them from effectively maneuvering in combat. It is most useful to ranged classes when fighting opponents who only possess melee attacks, allowing them to kite the opponent around. Snares are also useful to prevent enemies from fleeing to safety or running for help (in instances especially).
Spells and abilities with a snare effect usually have a percentage listed (i.e. 
[Dazed] is a "50% snare"). The creature's full movement speed is reduced by this percentage.
Examples of abilities that cause a snare effect:
- Mage:
[Frostbolt], 
[Cone of Cold], 
[Blast Wave] - Rogue: [Crippling Poison]
- Warrior: [Hamstring], [Piercing Howl]
- Hunter: [Wing Clip], [Frost Trap], [Concussive Shot]
- Shaman: [Frost Shock], [Earthbind Totem]
- Priest: [Mind Flay]
- Warlock: [Curse of Exhaustion]
- Paladin: [Seal of Justice]
Free Action Potions make you immune to snare (as well as stun and root) effects for 30 seconds.
See also: Crowd Control, Charm, [Dazed], Fear, Root, Sleep, 
[Slow], and Stun.