Boarding (gameplay mechanic)
From Halopedia, the Halo wiki
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, also called carjacking or hijacking, 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.
In Multiplayer, a successful boarding of a ground vehicle will award the player the Highjacker! File:Hijacker.JPG medal. If the player boards an aircraft the Skyjacker! 
medal is awarded instead.
Boarding Certain Vehicles
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. The Ghost and Chopper vehicles are the easiest to board because they only have forward firing guns so they're safe to approach from all other sides.
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 can be difficult to get close enough to board them, and because you cannot safely board a vehicle while there is an enemy in the turret. However if the player does get the opportunity he/she can render the vehicle useless by 1) ejecting the driver and thus the vehicle becomes a sitting target or 2) ejecting the gunner so that it is no longer a threat to other players.
Scorpion/Wraith
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).
List of Characters able to Board
You are not the only one that can board vehicles. In Multiplayer or Campaign if you are not careful you may find your vehicle hijacked by these enemies:
