Turn based strategy games, often abbreviated as TBS, are a form of strategy game that is not using a real-time mechanics. Every move can take as long as the player wants to take for it. Once the player has done all available and desired moves, he ends his turn and the opponents turn starts. While a player is moving, his opponent cannot take action.
The advantage is, that a player has the time he needs to properly plan his moves and strategy. This allows for an easier access to a deeper play. The disadvantage is, that the game usually takes longer. This is especially problematic for games, where a human player has to wait for his opponent to finish his turn (e.g. UFO2000). This has been partially remedied by some games, where multiple players can form a team. Not all players are playing simultaneously and not all players are playing one by one. The teams are playing each turn. Once all players of a team have finished their turns, the next team's turn begins (e.g. Birth of the Federation).