I am a big fan of World Of Warcraft, been playing it for over 3 years now.
One of the recent changes within the game is that the game is less about just attacking creatures but more about which spell or action to take. One of the actions is to crowd control monsters, which means to incapacitate them for a short time whilst you take down their friends. Which means you take less damage and the group as a whole can get the job done quicker.
For a quick reference of what is the best way to crowd control I have created a spreadsheet to help new people look up: