There are three Jeep-type vehicles, each with a machinegun mounted on them and a seat for a third passenger who can fire any of his weapons from that position - yes, including a Bazooka. For all intents and purposes they are the same except for looks.
Of course, there are also the tanks. Five make their appearance. To be more precise, there are three tanks and two heavy tank-destroyers. America has its venerable Sherman with the 75mm gun, Germany has the Pz IVH and Russia has its T-34 with the 85mm gun and corresponding turret. However, despite the historical differences between the tanks, for purposes of game balance they've been made identical. They fire the same shot which has probably the same power as the Panzerfaust, though it flies much faster and straighter. They have perfectly similar armor and speed. Thus, the differences are merely cosmetic. The three tanks can have a second crew member who mans the top heavy machine gun, and as additional defense against infantry they have light machine guns installed in the turret. This gun is activated by hitting the melee attack key.
Besides the three main tanks, there are two heavy tank destroyers. The Soviets are armed with the SU-152, technically an assault gun, but in the game it does serve as a tank destroyer. This huge lumbering beast, like its German counterpart the Elefant, has no turret but a devastatingly powerful gun, unmatched armor and is incredibly slow. Infantry armed with anti-tank weapons are the worst nightmare of these two titans since they have almost no defense against them, as they have no turret to turn quickly and machinegun down the infantry - though they do have a forward-facing machine gun, nor do they have the option of a top machine gunner.
The tanks are steered much as in Battlefield 1942, but in a 3rd person view. The aim-down-sight key, usually the right mouse button, switches to a view down the main barrel. This does away with the option of steering the tank independently of the main turret, narrows the view but does improve zoom.
United Offensive's tanks use a hitpoint system, not physics, to determine damage. No amount of rifle, machine gun or SMG rounds will hurt a tank, but on the other hand, even a glancing hit by a T-34 on a SU-152's frontal armor is guaranteed to do damage. While this may tick off the grognards, it does make for easy and fair game balance. As is standard practice, tanks take the least amount of damage on their frontal areas, more on the side and they're guaranteed to brew up with one hit in the rear. The Elefant and SU-152 are of course more resistant to the lighter tanks' attacks.
Enemy tanks can be hijacked as long as they're not being controlled by an enemy player. Hijacking does take quite a bit of time and makes a racket as the hijacker-to-be grinds away with the engine. The only other way to tell that a tank's being hijacked is to approach and try to enter, only to be denied access.
One last thing: you'll never, ever run out of tanks. There are so many tanks that even with 30 players on the server, if we lacked anything it was infantry. On top of that, all vehicles respawn quickly. Controlling an enemy tank spawning ground is quite difficult then, as you can imagine.