Boarding a vehicle is a technique featured in Halo 2 and Halo 3. It is a unique and interesting way of removing the threat of an enemy vehicle while not destroying it so that the player may use it as their own.
Summary
Boarding a vehicle is accomplished simply by pressing X in Halo 2 and RB in Halo 3 when the player is near an enemy's vehicle. This is not easily accomplished as the player can either be run over or killed by the vehicles gunner. However if the player succeeds he/she can instantly turn the tables on the enemy by using their ex-vehicle against them.