How tournaments differ from ring games

In tournaments the stakes are raised on a regular basis and that forces players to take part in the action. If you just sit and wait your stack will slowly be eaten up by the blinds or antes.
Since the tables are broken up as players get knocked out you will often face new opponents during a tournament.
All-ins before the flop is much more frequent in tournaments than in ring games.
The chips change in value in tournaments, since the prize pool is divided between several players.
When you play in a tournament you will know in advance how much you’re risking to lose (the exception to this is re-buy tournaments).
Tournaments have several different stages that require different strategies.