Game Mode: Enemy drones are spawned at regular intervals and you need to destroy them while making your way through the level, destroying the portals they come through and collecting ammo. At the end of the level there is a surprise.

Survival Mode: Start on an open battle field. Drones and Battle Cruisers are spawned at ever decreasing intervals. See how long you can last before you are overwhelmed and destroyed.

Weapons: You can pick up a variety of weapons in the game in addition to your starting weapon. Look for the long rectangular crates that store weapons.
Grenade Launcher
Machine Gun
Laser
Rocket Launcher
Guided Rocket Launcher
Capture Gun
Plasma Cannon
Cluster Launcher
Sticky Mines
Ball Gun