A time control dictates how much time each side has in a game of chess. It is written in the format of (Number of moves)/(Number of minutes.) Some popular time controls are:

G/30 - Game in 30 minutes (for each side.) This is "fast" for standard games. Game will last an hour max.

G/65 - The tournaments I have played in have used this TC. Nice and roomy.

G/5 - Normal blitz chess TC.

G/3 - Even faster blitz chess (:

Some digital chess clocks allow for an increment. An increment is an amount of time, in seconds, that is added to a player every time he makes a move. Another popular time control is 2 minutes each side with a 12 second increment.

I believe higher level players like to use TCs such as 30/60 SD/30. This means you have an hour to make your first 30 moves, then those 30 moves you have a sudden death time of 30 minutes added to your clock. Note that this is completely conjecture as I am too much of a patzer to know what higher level players do (: