Crowd Control is just terminology that refers to a skill or set of skills that allow you to "control" a battlefield, or enemies on it.
Using certain skills or spells, you can deny enemies the ability to act or do damage. Some examples would be:
Crushing Prison (spell): Will root an enemy to the spot and render him unable to act or move whilst taking some pretty heavy damage. This effectively "removes" him from the fight for the duration of the spell.
Force Field (Spell): Will encase a friendly player and make them immune to any damage, but also prevent them from moving or attacking/casting spells (they basically become frozen). Again, removing them from the fight for the duration. Happily, you can also cast it upon enemies. You wont be able to do any damage to that enemy, but you remove him from the fight so he cant do any damage to you (until the spell runs out)
Petrify (spell): Turns an enemy to stone, thus removing them from the fight for the duration of the spell (as they cannot act).
You can also chain these spells if you wanted. For example, you could keep a boss out of a fight until all his lackeys are dead by casting Petrify, then when it runs out, crushing prison, then when that runs out, force field. That would basically give you 2 minutes of boss free action. Remember, they CAN resist spells so you can't always rely on it to work perfectly
There are also shorter versions of Crowd Control, for example a rogues Stun skills. Dirty Fighting is a good way to briefly disable an enemy (especially while it's casting a damned fireball!) allowing you to position yourself behind it. Riposte is another good one. I also put Glyphs of Paralysis in both my weapons.
So yes. Basically, any spell or skill that allows you to control (ie: prevent or delay) the movements and actions of an enemy could be referred to as Crowd Control, as effectively you are dictating what you will allow to happen.