Pressure

Pressuring
Pressuring means directly challenging the ball carrier with the purpose of regaining possession. It is a tactically aggressive act that may cause problems in situations when your defense is inferior in terms of numbers. In theory, pressure makes it more difficult for the enemy to operate the ball. That is why it should be applied more firmly around one's own goal - it narrows the enemy's chance of scoring. Applying pressure could also be used to regain possession of the ball. If the defending team has better/more aggressive one-on-one specialists they may try gaining control of the ball right away. Conversely, when playing against a squad of skillful dribblers, it would be wiser to challenge for the ball only after getting adequate backup. Where to pressure is another important factor that must be coordinated in a tactically sharp team. Pressuring with the intent of gaining possession requires a good amount of numbers and proper marking. Without shutting down the off-ball opposition there will be little effect in pressuring. Ideally, a tight defense would force the ball back/outside when applying pressure.

Backup
Providing backup, or cover, means having an additional defender behind the player who is pressuring the ball. This discourages the attacker from trying to beat whoever is pressuring him and forces him to pass the ball sideways.