A string bet, also sometimes referred to as a string raise, is a common violation of poker rules. It is usually done in good faith, but unethical poker players definitely use the string bet in an attempt to gain an unfair advantage over their opponents.
The string bet occurs when a player bets or raises in a motion that is not fluid, and does not announce to the table that he intends to bet or raise. If a player reaches for chips, moves them across the line, then hesitates and reaches back for more chips it is a string bet. In limit games the following scenario is the most typical for string bets: A player initially puts in chips that represent a call. He then moves to add more chips, but because he did not first say he was raising the move is illegal.
It is always a good idea to announce to the dealer and the table at large when you are going to bet or raise. That way you can move chips around back and forth before committing an amount to the pot.
One of the many advantages of online poker is that it eliminates the possibility of a string bet. Just as most poker software makes it impossible for the winning hand to be accidentally mucked, it also ensures that the betting process is far smoother.