Root
Root effects are a form of crowd control which cause the victim to be locked in place, unable to move. Root effects are sometimes known as immobilize effects in-game. A root effect differs from a stun effect in that the target is still able to use abilities, spells and melee/ranged attacks, but cannot move from its location as long as the effect is active.
Many root effects are subject to diminishing returns. Roots are movement impairing effects, and so can be countered with effects such as 
[Hand of Freedom].
Notes
Roots are primarily used to prevent targets from escaping; or to prevent targets from dealing melee damage by keeping them out of melee range. Roots therefore tend to be of little use between two ranged types, or between two melee types. Roots are generally far more important and powerful abilities in PvP, where their proper use can be critical. The most common use of roots in PvE is to prevent enemies from fleeing when their health gets low, preventing them from aggroing other mobs.
Roots are useful to melee classes such as warriors seeking to get close to ranged classes such as mages; and useful to ranged classes such as mages seeking to evade melee classes such as warriors. Since melee damage depends on being in close proximity to the target, the proper use of roots (along with snares, stuns and similar effects) can make the difference between victory and defeat. Roots and snares are critical for kiting, a technique primarily employed by mages and hunters to keep melee enemies out of effective range while the player continues to deal damage to them.
More specific uses of roots include preventing enemy players from moving out of line of sight, where they could hide from attacks, heal or drink; preventing a healer from moving into casting range of an ally in need of healing; and slowing the progress of flag carriers in battlegrounds.
Note that if a mob with no ranged attacks is rooted from range, it will be unable to attack its current aggro target. However, if players are standing in melee range, it will attack the player that has the highest threat level. Thus, it is not wise to stand near a rooted mob, or root one within a crowd of players without prior warning.
Abilities
The following are some examples of abilities and procs that can root targets:
- Druid:
[Entangling Roots] - Mage:
[Frost Nova] - Monk:
[Disable] - Warrior:
[Warbringer]
Counters
Items which can dispel root effects:
Some examples of class and race abilities which can dispel root effects:
- Human: Can use
[Will to Survive] to remove all movement impairing effects (which includes immobilize effects). - Hunter: Pets under
[Bestial Wrath] dispel initial root effects. 
[Master's Call] will also remove all roots and snares. - Paladin: Can apply a [Hand of Freedom], providing a brief immunity to root effects.
- Priests: Shadow priests can use
[Dispersion] to break free of all roots and snares, and gain immunity to them for 6 seconds. 
[Phantasm] allows 
[Fade] to be used to similar effect. - Mage: Can use
[Blink] to free themselves from root effects. - Monk: Can use
[Nimble Brew] to free themselves from root effects and reduce the duration of further applications by 60% for 6 seconds. - Rogue: Subtlety rogues can use
[Shadowstep] to remove root effects. - Warrior: Can use
[Bladestorm] free themselves from root effects and become immune to them for the duration. - Gnome: Can use
[Escape Artist] to free themselves from root effects. - Druids: Can Shapeshift to free themselves from root effects.
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