In each game you start in a random galaxy with several races. Each race's goal is to conquer the galaxy, the problem is the other races are in the way. This is the plot of many strategy games like Master of Orion or Civilization (well Civ is planet bound).
These races expand their territory by scouting out and colonizing suitable planets. They build up their planets with more and better buildings that enhance their economy. They go to war and attack enemy races to hurt them or take their planets. They do much of this by building fleets of starships. They also research new and better technology to build better buildings and ships.
You aren't in control of any of this, directly at least. You are a mercenary in this very dynamic galaxy. Your job is really to survive and make a good living in the meantime. You can pick the side that you feel is right and help them conquer the galaxy for its own good. You can pick the side that you think is going to win and go along for the ride. You can even try to play the races against each other and simply try to pick the winning side at the last second.
You might just be a small mercenary ship but you have a lot of influence in the direction of the galaxy. You can scout out suitable planets to be colonized. You can attack and destroy defenses. You can find and possibly even steal technology to give to the race of your choice. You can even find items and arm whoever you want, including yourself.
At the very beginning you start with a small escort ship, but every so many levels you can upgrade to a larger ship like a frigate, destroyer, cruiser, battleship, all of the way up to a flagship.
