Kerbal Space Program is a space flight simulator that gives you the task of building a rocket capable of taking a crew to the moon. It has very realistic physics and is surprisingly difficult. However, this is softened by being set on the fictional planet of Kerbin, populated by a cute intelligent alien race called the Kerbals. It has an extensive career mode, and a sandbox mode that gives you total freedom to experiment.
Space, the final frontier
Before sending anything into space, you first have to build your rocket. Starting in a giant hangar, you have to piece together your first rocket from parts you find there. It doesn't take long to reach your first lift-off, but you'll have a long way to go to achieve your aim.
In sandbox mode, there are no limits to the design of your rocket. You can do what you want, but your rocket will still conform to physics, so they will often result in spectacular and explosive failure.
You won't find out until launch whether or not your rocket will be successful. Hit the space bar, and it will hopefully go through the various stages you defined on its way to space. Or it might not.
In Career mode, you progress through missions, with a tight budget. You'll research new technologies in the the Space Center, learning from your successes and failures, training astronauts and so on.
Almost as challenging as reality
The cuteness of the Kerbals shouldn't make you think there is anything easy about Kerbal Space Program. From the moment you make your first rocket, until you finally send a crew of astronauts successfully to the moon, the difficulty is pretty high.
Kerbal Space Program is tough because it's realistic, like every good simulator. Achieving escape velocity is relatively easy, but finding a stable orbit is a whole other challenge. Even managing this, you still have a lot to overcome before you can land on one a planet. It takes hours and hours of dedication, but it's a rewarding experience.
If you're discouraged by Kerbal Space Program's immediate difficulty, do not despair. There's an official wiki page with a wealth of help and tips to make sure your space program is going in the right direction. There's an excellent and enthusiastic community of players, all willing to share their tips and tricks.
A dream that requires patience
If you want to feel like a NASA engineer, Kerbal Space Program is the game for you. Its complexity allows you embark on a space race that lets you learn a lot about building rockets, provided you have the time and dedication!