What's the difference between a tournament and a cash game?

The main difference between cash games and tournaments is that in a cash game you can join at any time and choose how much you bring to the table and leave at any time with however many chips you have at that point.

In a tournament, all the players start the game at the same time, with the same amount of chips, and it only ends when there is one winner who has all the chips.

Also, cash games are only played on one table but tournaments can be played over many tables. The blinds rise incrementally in a tournament but stay the same in a cash game.