Crowd control is a term and tool used in several
mmorpgs. It refers to any ability, or the effect of any ability, that restricts the movement of a player or group of players. What crowd control means to many
DAOC players is standing helplessly, under the effects of a
mesmerization (mez) spell, watching teammates be slaughtered one by one and waiting for the mez timer to tick down. Suffice to say, crowd control in
DAOC is very powerful.
There are several types of crowd control, based on what they do, and how they can break.
A mez generally refers to an ability that prevents a character from acting in any way, but is broken when the character recieves a negative effect, or is hurt. An example would be the mage's polymorph in World of Warcraft. Mezzes are usually of a very long duration, taking opponents out of the fight until their allies are already dead.
A stun also refers to an ability that removes a character's ability to act, but also lasts its full duration regardless of whether the character is struck. Stuns are of much shorter duration than mezzes, but the fact that they will not break means the stunner can beat on the stunned character while the latter is unable to defend himself.
A root refers to an ability that prevents a character from moving at all. How effective this is depends on who is rooted. An archer or mage won't care, they'll just keep chucking stuff at you, unless, that is, they have someone smacking them at close range while they can't run away. Rooted warriors, however, will shake with rage as you stand just outside their range and taunt them. Look at that silly clubswinger, immobile and helpless. Sucks to be the one who's taking the hits while you can't do anything, huh?
A snare is the weakest form of crowd control, but it can be effective as well. It slows the target, but does not stop its movement, letting him be caught and slaughtered, or kited (slowed enough that the snarer has space and time to cast spells and such alternating with running) and kept out of reach of the caster or his allies.
Crowd control is one of those things, much like sneaky thieves, that are lots of fun for the person on the right side of them, but not so fun for the guy who is killed without having a chance to defend himself. In DAOC, crowd control is in place because of the prelevalance of zergs - massive parties of players travelling in one big group. It evens the playing field a bit, and gives smaller groups a chance to not be overwhelmed by sheer weight of numbers.