In short, you take control of either the Klingons, the Federation or the Romulans in 14 missions to colonize strange, new worlds in an area known as the Tabula Rasa, located in the Neutral Zone. During this, you are in control of your colony, and your responsibility is, aside from completing the missions is to build up your base, provide it with enough power to let all systems run flawlessly, have minerals processed to gain new materials you can use for building/manufacturing and make sure your colony is strong enough to defend itself against enemy forces.
A screenshot:

This is an example of my Klingon Base on the planet AP173.B, complete with assistance from a Bird-Of-Prey





