Definition
The prior posts have been good, but incomplete.
The term set usually refers racing to win a certain number of games. Back in the day, you would race to 5 for $50, 7 for $100, 9 for $200, and I shot several sets to 11 for $500 and $1,000. Whether you have to win by 2 games is dependent on the 2 players playing. Today, any sets played for more than $100 are usually longer sets than in the past. For instance, $200 or $300 sets might go to 11 or 13, $500 sets might go to 15, and $1,000 sets might go to 21. It just depends on what the 2 players playing decide.
Then what noone has mentioned before is 'ahead sets'. This is where 2 players play until one player gets a certain number of games ahead of the other player. For instance, an ahead set to 6 would be over when 1 player got 6 games ahead of the other player. Ahead sets, though, can take forever to play if 2 players are pretty evenly matched. I quit playing ahead sets long ago after spending over 16 hours once playing an ahead set, for one set.