Immobilize (or Root) is a type of crowd control that involves preventing an opponent from moving.
Immobilizes effects come in two main categories, activated and random trigger, which are on separate diminishing returns timers. Nevertheless, all Immobilize effects are all functionally identical; they prevent the target from moving for the duration of the effect, but do not prevent the target from acting. Immobilize effects do not break automatically break on damage, but attacking an immobilized target does give them a heightened chance to cause them to break free.
Activated Immobilize effects include Druid's Entangling Roots, Mage's Frost Nova, Hunter's Counterattack. Random trigger Immobilize effects include Mage's Frostbite, Hunter's Entrapment and Warrior's Improved Hamstring.
Immobilize effects can be removed with a PvP Trinket, as well as Mage's Blink, Rogue's Shadowstep and the Gnome racial ability Escape Artist. Hunter's with the effects of The Beast Within and anyone under the effect of Paladin's Blessing of Freedom, Free Action Potion and Living Action Potion are also immune to Incapacitate effects for the duration of the effect. Additionally, because you are still free to act under the effects of Immobilize some classes can dispel the effect off themselves without needing immunity.