The game is played over a series of missions, three of which are included with the program. More missions are being worked on as you read this, by myself and others. And if you don't like these missions, there's no reason for you not to create your own!
On payment of a small shareware fee (more information later on) you'll get the mission designer that I use, which makes mission creation very simple. Everything from the map, to the weapons, to the sound effects can be changed.