How spaceships travel through the space?

There is no gravity and oxygen in the space and I believe it is gravity or something that allows air transportation move. For example planes do not float in the air like spaceships but are controlled very well. But in the space the spaceships seems to be simply floating as there is no gravity, no air for fuel burning. So how the spaceships can come back into the earth's atmosphere? what are the driving forces in the space?