KSP is a game developed by Squad that lets you construct 'craft' which you then attempt to pilot into space and to various worlds in the Kerbol solar system. How you create your craft is totally up to you and your imagination is the only limit...well, that and the laws of physics, which you'll end up accidentally learning quite a lot about as you try to get your creation to take flight.

All the craft on this website (which incidentally is a fan made site, not affiliated with Squad) are built in KSP. Some of them just use the core game and some of them also use community made mods (which are all helpfully listed along with each craft).

Those of us who love KSP see it as much more than just a game. It's a canvas that lets your be as creative as you like while also providing a great learning resource for understanding the mechanics of space travel.