Let's say your group rounds the corner in Heroic Stonecore. You see a group of five humanoids and elementals chatting away. The tank slaps some raid icons on the targets. The dps know exactly what their job is and the group of five becomes a banished elemental, harmless sheep, a flailing human, and two rampaging warriors. Sounds better then all five charging up their lasers right? That's the power of crowd control. Your group (and raid) will be all the better off because of it. There is a chain of fortunate events that take place with CC. Now the tank is only being hit by two targets so he isn't raking too much damage. Since there is not that much damage the healer doesn't have to expend as much mana. And since there and only two active targets wailing on the tank the dps can focus their fire and kill two quickly rather than five slowly. Now let's say that your group doesn't have the Druid, Mage, and Warlock from the example group. You can still survive. Even using just one CC will help out to mitigate damage or prevent potent abilities such as AoEs and heals. Tanks and group leaders should familiarize themselves with all the classes and their specific CC abilities. As the Warlock banish can only be used on elementals and Polymorph can only be used on humanoids and beasts. While Hunter traps are far more versatile.
Announce your crowd control target
---Making a macro that /says your incoming CC is a good idea.
---Tell your group which CC you have available before you start.
---Determine which party member will call out/mark targets for which CC.
Choose wise targets
---Pick a target that is not in AoE range of the tank (typically casters/ranged targets)
---Usually CC the DPS targets as healing targets that have their CC broken can make the fight longer. Kill the healers first.
---Using a line of sight (LOS) pull can help separate the CC target from the rest of the mob.
---You can cast the CC first and have the tank pull the incoming adds off as they round the corner -or-
---Have the tank aggro the group and pick off a caster as they start their spell.
---This choice is dependent on your group & play style as well as the tank's experience.
Some CC fully immobilize the target (Tornado, Polymorph, Hex) while some are movement restricting (Frost Nova, Ice Trap). Be careful when uses fears and psychic scream as targets have the tendency to pull nearby groups. This isn't too much the case in Cataclysm dungeons but always be cautious.