Poker tournaments differ greatly from cash games. In cash games, the winner can immediately claim his prize and live the game. It is also allowed to have as many chips as you require, or buy additional chips.
In tournaments, the number of chips is set by the rules, and to exchange his chips for money, a player has to reach a certain stage of the tournament.

The strategy and phases of poker tournaments

Early stage – it is not advised to be aggressive at this time. Early on, you may encounter a large number of novice opponents, who will frequently bluff, call every stake, and act impulsively. You should play defensively against them.
Medium stage – your primary objective during this stage is to build your stack as much as feasible. It is preferable to minimize unnecessary risks, such as entering a draw without the raise. For this stage, aggressive, tight playing is recommended.
At this time, all players believe victory is imminent and attempt to play extremely cautiously. This permits violence, such as collecting antes and stealing blinds.
In the final level, you will face the finest players, who likely have years of expertise. The finest piece of advise is to remain calm and concentrate on the game itself, rather than the money you may earn. Playing against experienced opponents who will adapt and change their plans makes it difficult to select a single approach that will work properly at this point.