The Warthog is the jeep-like vehicle in the Halo series. Appearing since the very beginning, the Warthog is one of the most iconic vehicles in the series.
While it was decent in multiplayer, I didn't care for it during the campaign of the original. I never liked being the driver since it steers like junk and you can't fire a weapon at the same time. If you're playing solo, then only CPU players can man the turret.
Which is something they fixed in Halo 2 and onward. It was pretty much the same in multiplayer in both Halo 2 and Halo 3. Depending on the map it could either be pretty helpful or one of the most unwanted vehicles. A good team of Warthog partners can be quite a challenge if on the other team.
I really liked the rocket-equipped Warthogs in Halo: Reach. Those were nothing to joke with.