Action points are gained through completing mission objectives, taking down foes and capturing enemy positions. The main objective, at least in the single-player mission we played, is to move through the landscape and capture and hold various control points throughout the map, similar to Battlefield. If there's a particularly well-defended control point, you can spend some action points to call in an artillery strike or air strike. After you capture a point, you can reinforce the area with fixed machine gun and anti-tank emplacements, which you can man yourself to take out waves of enemies.
You can also buy vehicles such as APCs, jeeps and tanks (in several different flavors, including plain vanilla and heat-licking flame) using, you guessed it, action points. You can either opt to drive the vehicles yourself or man the gun while one of your squad members takes the wheel. There are nine vehicles for each side -- Allies and Axis (yes, you can steal enemy vehicles) -- with different variations of each. You can also drive civilian vehicles, but they aren't nearly as fun since they don't have any blazing guns attached.