The small blind is the position between the button and the big blind.
The player in the small blind must make a forced bet before even seeing his hole cards. He is always first to act after the flop and will be out of position for the duration of the hand, which is why this position is considered the worst at the table by many players.
This small blind is usually half the size of the big blind.
For example, in a $5/$10 cash game, the player in the small blind position must place a $5 bet at the start of the game. The blinds move one seat clockwise so that every player can eventually play from the small blind position once in every orbit.