Although there are only 3 types of vehicles and they only appear on 2 mutliplayer levels (Sidewinder and Blood Gulch), it is important to know each vehicles characteristics before jumping into one. Each vehicle has infinite ammo and will instantly kill anyone who is run over.
A very fast vehicle. The mean lean purple machine can hover around levels faster than any other vehicle (but its not the fastestest) and is the only vehicle capable of strafing. Use this ability to fight tanks and run over pesky enemies. It's dual cannons fire rapidly and are good at close to medium range. The ghost gives its rider very little protection but it is compact (good for traveling in tunnels and bases) and is not effected by the ice in Sidewinder. Use it to quickly get to flags or bases. Pratice turning with this vehicle because it tends to slide. Also it is possible to turn and fire your guns before your vehicle is actually facing that direction. Your enemies will probally hear you coming so watch out. The ghost has a very distinct sound. Watch out for other vehicles or rocks, otherwise you'll go flying an you might land upside down.
Tank (M808B Scorpion MB)
A slow moving vehicle that is capable of killing anyone in with a single blast of its mighty cannon (even with an over shield, your dead meat). Anyone in its path is in danger. No vehicle can protect an enemy from this blast, not even another tank. The cannon seems to have an infinite amound of range and the blast it creates is huge. The tank is huge and slow moving, which can make it an easy target for a good sniper or someone with a rocket launcher. It is also very loud which warns other players you are coming. Try standing still and then surprising a sucker when he comes around the corner. If someone gets to close or your waiting the 4 seconds it takes to reload the cannon you can use its machine gun fire to take out enemies quickly or you can run them off with your massive size. The tank also has the ability to hold 2 other players per side, giving it a total carrrying capacity of 5 players. The tank is deadly, but it may also cause you to become the target of everyone else on the other team. The tank is not easily turned over but it is possible.
Warthog (M12 LRV)
Controlling this "jeep" takes some getting used to as it tends to slide and roll over easily. Its the fastest vehicle in the game (in a straight run). Mounted in the back is a fully rotatable 3 barrelled turret which another player can climb onto. Another player can also jump into the passenger side for added firepower. If your going out by yourself, use the ghost. If your going out as a group, the Warthog is perfect for some major destruction. Slide to the left to allow both the passenger and the man operating the turret to fire on the same target simultaneously. The front of the Warthog provides the most coverage against fire, so drive right in there and kick some ass. Running over enemies can be difficult in the Warthog as it tends to powerslide in place.