You control the little green guys above, the Kerbals, who live on a planet not unlike our own. The Kerbol system currently has 7 planets and 9 moons (the majority of which are above Jool). At this stage the goal of the game is whatever you make it, so much like Minecraft, you set a goal and then work toward it however you can.
Obviously this game has rockets (otherwise it'd be pretty hard to get to space), and to make them you take individual parts and snap them together to create a rocket. According to the wiki there are currently 128 parts in the game and they range from rocket and jet engines to probe control modules and the aforementioned rover wheels. You can make epic space stations, or just fly a one man capsule to the Mun and bounce around on it.
|Jebadiah... Why did that chute deploy?!|
However the thing I find most fun about this game is the designing of rockets. There are some elegant solutions to even the most complex of problems and I have lost literally hours trying to figure out how to get something into space in one piece. My current mission is to get a rover and lander module into orbit without two rockets and I have spent maybe 3 hours so far changing designs to see if it can be done. So far no luck but there have been some spectacular explosions.
Anyway as mentioned the game is still in alpha but seems to be pretty stable from what I've seen. If real world space travel interests you, I'd recommend picking it up to have a look.