Capturing is used on enemy points to claim them as your own. When standing next to an enemy point, hold down E (default key) to start capturing. The capture bar will need to fill up completely for the capture to be successful. While capturing, you will be interrupted easily by melee.
Every race has 1-2 classes that hold a Capture Key. A Capture Key allows you to capture enemy points. The following is a list of the classes with a Capture Key:
Chaos Space Marines (2): Traitor, Traitor Assault
Eldar (1): Dire Avenger
Space Marines (1): Tactical
Orks (2): Shoota Boy, Slugga Boy
Interrupting a capture:
To interrupt a capture, you can do one of two things:
A) Kill the capturing enemy.
B) Melee attack the capturing enemy.
Both will stop the capture process and the enemy will need to start over to capture the point. While melee is typically seen as easier to accomplish, it will require close proximity to the capturing enemy. Whereas killing them with a ranged weapon or a grenade does not require you to be near the them.