Summary
The Scorpion is the human tank so named for its shape, with its main gun resembling a scorpion's stinger. It has a short-to-medium-range machine gun and one powerful cannon. It can seat one driver and four passengers. While the main cannon can only shoot one round every few seconds, the machine gun has unlimited ammo, and can mow down troops while the cannon is reloading. In Halo 1, the main cannon takes four seconds to reload.
Combat
While the Scorpion can sit four passengers, in Halo 2, you as either playable character cannot board it. In Halo 2 multiplayer, the Scorpion is strictly single-user as allies still cannot board it. Passengers of the Scorpion tank can fire whatever weapons they are carrying in the same fashion as the Warthog, but are much more exposed to danger than Warthog passengers.
To hijack a Scorpion, one must melee out the hatch and kill the driver, just like with a Wraith.