Ghost
This small, one-passenger craft is a handy tool in Halo 3, but you probably won't find unmanned Ghosts very often. When you do climb into one, you can use it to move a bit faster, take a bit more damage, and fire on your enemies, so you're basically just a bit more enhanced in your combat capabilities than you would be on foot.
That said, the Ghost is a fun vehicle when you can get in it, as it's quick to move and can deal good damage with its dual plasma cannons. You can tap the left trigger for a burst of speed, which is handy if an enemy is directly in front of you and you want to run them down. Just be careful, as the Ghost is prone to tipping on bumpy ground.
Wraith
The Covenent's heavy tank is the Wraith, which has been overhauled in Halo 3. In Halo 2, it was usually pretty easy to jack these vehicles, but the vehicle has been retrofitted with a forward-firing turret in H3, making it a much more deadly vehicle at all ranges.
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Taking out a Wraith is a tall order now. In Halo 2, you could simply run up to them and jack them, but the presence of a turret on their fronts makes that very difficult. In the campaign, you'll usually have access to a turreted Warthog or another long-range vehicle that you can use to take out a Wraith, but if you don't, you might want to try sniping out the turret gunner (he's vulnerable to headshots), then driving a quick vehicle up close and jacking or grenading the Wraith.
Mortar Variant
Most of the Wraiths you see will pack mortars that fire huge bursts of energy at extremely long ranges. These move very slowly, in an arcing manner, so you should be able to see them and dodge them when you're at a distance. As you get closer, though, these mortars get progressively more deadly, since they can be fired at very close range and will impact pretty much instantly. On Heroic difficulty or higher, a single mortar hit anywhere near you will instantly kill you, and they can hit even quick vehicles pretty accurately if you're driving straight towards them. Try to approach at an angle, then, and if possible get on their rear side, where you can grab onto their back sides and throw a grenade into the engine without having to deal with the turreteer.
Anti-Air Wraith
There's only one part of the game where the AA Wraith will actually be firing on you while you're in an aerial vehicle, and even then you're not in much danger; these guys are more intended to be used against Pelicans than Hornets. They won't be able to hit you with rockets, although some may home in on you if you get too close to the stream.
AA Wraiths can be fired at ground targets, and this is tough to deal with since there's so much fire that comes your way. Best to fire on them from a distance, or use quick vehicles to make your approach. These guys have turret gunners, just like the mortars do, though, so be careful.
Chopper
The Chopper is the new one-seater vehicle in Halo 3. If the Ghost is the small and sleek vehicle of the Grunt, then the Chopper is the powerful motorcycle of the Brutes, who will almost always be the ones you see driving it. There's nothing especially complicated about it; the primary fire button is a powerful laser cannon that can chew through most light vehicles relatively quickly, while the left trigger will get you a bit of a boost in speed for a few seconds.
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The narrow width of the Chopper will make it difficult to hit players with it, unless you're good at fishtailing the vehicle. The width also makes it easier to get into lengthy flip sequences when you get off-axis, though, so you need to be careful while driving.
Prowler
The Prowler essentially replaces the Spectre from Halo and Halo 2; instead of being a vehicle designed to take Elites into battle, the Prowler…is a vehicle designed to take Brutes into battle. As such, it's much more aggressive in form, and seems to be more heavily armored. In addition to the driver, there's room for two soldiers as passengers on the skids and one as a gunner. This is the closest analog the Covenent forces have to a Warthog, and like that vehicle, won't be very useful to you as anything other than transportation unless you can find a friend to ride with you.
Banshee
The ever-popular Banshee returns in Halo 3, and this time around it has a new addition to its arsenal: a homing missile. It doesn't seem to do as much damage as the homing missiles on the Hornet, but still, it'll help you out in those wild dogfights.
