Various Spells and Special Abilities are able to disable enemy attacks and/or movement; in the latter case, melee units' attacks will be neutralized if they are unable to close to melee range. Movement neutralization is also useful for avoiding enemies altogether, which is especially useful in Player vs Environment maps.
In the final analysis, Crowd Control's purpose is damage reduction. Crowd control reduces the number of damage-dealing targets, making large numbers of enemies more manageable.
In alphabetical order, the main forms of crowd control are: