The term spot fixing is used when a player in the game agrees to affect a particular part of the game in exchange for money. It is dependant on the performance of one or two players and usually does not affect the outcome of the entire match.
In cricket, this agreement is usually for an over or a ball.
For example, in a particular over a bowler may agree to bowl a no ball or a wide or a batsman may agree to lose his wicket. It could also be more complicated, as in a bowler may agree to give more than 12 runs in the over or batsman may agree to not play four balls of the over.
Since illegal bets are placed on every over, and every other aspect, of the game, spot fixing is done to influence the payouts in these bets.
Spot fixing differs from match fixing only in terms of scope. Match fixing, as the name implies, is when the entire outcome of the match is pre-agreed upon.
Match fixing also requires the participation of a large number of players and usually involves the captain of the team as well.
The pay offs in match fixing, and the risks, are higher.
Spot fixing is popular because it requires only one of two players and can be kept secret from the rest of the team. The payment to the players is also lower. And spot fixing is harder to catch since every bowl bowled or every ball played comes under suspicion and 99% of those are honest and fair.
The IPL particularly is considered vulnerable to spot fixing, since there are a large number of players and matches, with huge amounts of money bet on the matches.
Even in 2012, five players were suspended for spot fixing in the IPL.