Tournaments vs. Cash Games
The difference between a ring game and an online poker tournament is the way that money is added to the game. In a ring game, you play with the money you bring to the table and you can add more money to your chip stack at any time (but not more than the maximum buy-in) and reload should you lose all the money. To enter an online poker tournament, you pay a fixed price the tournament buy-in and you will receive a set amount of chips. Everyone in a tournament will receive the same amount of chips.
Another thing that is different between tournaments and ring games is that the blind levels will increase so that players are forced to make moves and to narrow down the field as the tournament goes on.
The chips in a tournament are not connected to money and you cannot trade the tournament chips for money at any time.