Well, supposedly, you could move around in space by sitting in a bucket and throwing rocks - you'd move in the opposite direction to the rocks you throw. It would be a very slow way to travel mind !
Thrusters work the same way, more or less, they just don't use rocks. What they use is compressed air or the gasses from combustion. Ummm ... if you blow up a balloon and let go, the air inside is forced out and the balloon goes shooting off. It's the same principle with spaceships.