Each character in the game has their own unique weapon, the Special, usually in keeping with that characters theme. The only way to collect ammo for your special is to drive over the green crates.
There are two distinct types of special, the on-off and the continuous. One-off specials are typically limited to 3 units (for example, John Torques' Bass Quake) amd require you to simply click fire when an enemy is targetted/in range. Continuous specials (for example, Sid Burns' Great Balls of Fire) need to be aimed in the direction of you opponent, but typically have a greater supply of ammo, in the region of 50.