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 vehicle's gunner. However if the player succeeds he/she can instantly turn the tables on the enemy by using their ex-vehicle against them. But be warned, the enemy can do the same to you, so make sure you don't give them the opportunity.
Boarding Certain Vehicles
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Ghost/Banshee/Chopper
The most common kind of boarding takes place when a player successfully boards a Banshee, Ghost, or Chopper an animation will take place showing their character model jump on top of the enemy vehicle and eject the current driver. Then the player's character will enter the vehicle and the player will now be the driver.
Warthog/Spectre
Boarding a warthog or spectre is different in that you still eject the other player but you do not enter his seat. This can be dangerous as the player that was boarded can start shooting you before you return to First-Person view. Because these vehicles also have a turret it is not likely you will get close enough to board them. However if the player does get the opportunity he can create a moment of confusion for the vehicles occupants as they wonder why 1) they are suddenly without a driver thus becoming a sitting target or 2) no longer have a gunner and are no longer a threat to other players.
Tanks
When a tank is boarded, instead of the driver being ejected a different event takes place. The player can repeatedly press B to melee the hatch off the tank. After the driver is exposed you can either keep meleeing until the driver is killed or, by pressing the Left Trigger, plant a Grenade that will instantly kill the driver. However if you plant a grenade you may also destroy the tank in the process.
Counter-Boarding Tactics
A player can avoid being boarded by doing one simple thing, constantly moving. However if you are boarded by getting out of the vehicle before you are ejected you may be able to re-board your vehicle. Additionally if you are boarded while inside a Banshee there is a maneuver that can be performed to recapture your vehicle (The Great Banshee Switch).