Definition of cbet
A cbet or "continuation bet" is a term introduced by Dan Harrington in his famous trilogy on poker tournaments. Basically, a cbet is a bet you make when you are the aggressor preflop. A cbet is a bet that the player who raised preflop can make (or the player who made the last aggressive preflop action). In this situation, we will say that the player has the lead. You have to have the lead to be able to make a cbet. If the pot is cleared, the bet will be an bet and not a cbet.
More details on the cbet
Generally, when a player has opened preflop and his opponent has called the bet, he will make what is called a cbet to flop. If the bet is called is that the player bets the turn, then it will be called a cbet turn. The majority of players make cbet's at too high a frequency. The mentality behind the cbet is "I raised preflop, I showed strength, you just called, so I have a better hand than you and I'm going to bet it (cbet) to flop". You can cbet on flop the turn and the river.