Kerbal Space Program (KSP) is a sandbox spacecraft simulation game where players build, launch, and control spacecraft and aircraft, and manage a space program as a whole.
There are three game modes to play in: sandbox, science and career. In the sandbox mode, all parts are unlocked from the start, allowing for player experimentation. In the science mode, parts are unlocked through a tech tree with science points, which can be earned through performing experiments and taking crew reports. In both cases, overall goals are player defined, and could include such activities as landing a Kerbonaut on the Mun and returning him safely to Kerbin, or exploring the other planets in the star system the Kerbals call home. The third mode, career mode, adds contracts, funds (ingame currency) and reputation. Contracts are mainly testing new parts or gathering science from orbit. You can gain funds and reputation by completing contracts, and sometimes even science points.
KSP is developed by Squad, who use a Minecraft-like business model to sell the game while it's still under development. The current version is available for purchase on their website or Steam platform for $30 (US), while an older version is available as a demo containing limited parts and celestial objects.
KSP also has an active modding community found on the official forums and the dedicated community parts site Kerbal Spaceport, who develop new parts and plugins. An officially hosted community wiki is extremely resourceful for new and experienced players alike.