Imagine of being the chief of your nation's space program: what would you do?
This is the basic concept behind Kerbal Space Program, developed by Squad and still in very early alpha stage: you build a rocket, decide what to do and then send your little Kerbals into space!
That looks simple right? Well you'll find out that this game has one of the steepest learning curves ever seen in a videogame and that it is an almost perfect spaceflight simulator that calculates trajectories and orbits using the principles of Astrodynamics.
After planning what will your mission be (which can differ from getting a rocket into orbit to making a permanent base on Duna, the equivalent of Mars into the game's solar system; basically, you can do whatever you want), you'll have to carefully construct and make separate stages of your rocket prototype considering what it's going to do and how far you want it to go. Once you have saved your new rocket you can do the countdown and fly into space.
Fortunately, the latest game versions include a number of tutorials that explain the basics of spaceflight and even explain you how to get to the Mün.
Even though the limitlessness of the sandbox gamemode and the great amount of community mods are already making this still-alpha game (which is surprisingly free of gamebreaking bugs) worth the money it costs, it's planned to include a campaign mode that is going to set objectives (the most important: get the astronauts back home still in one piece), money budgets, used parts recycling and more.
Right now, this game can take away lots of hours of gameplay that will go from moments of frustration when you make mistakes, to moments of happiness when you accomplish your tasks. Playing KSP may get you an interest into Astrodynamics or give you an amazingly realistic playground to test your theories if you are already one. Needless to say that the learning curve is so steep that this game is just not for everyone willing to fly things around space with no reasoning behind that, and that fully grasping the concepts of space travel is really hard and takes a lot of time.