To begin, I guess I should give a short description: Essentially, KSP is a fairly realistic simulation game where you take control of and guide the nascent space program of a race of small green men and women from a planet named Kerbin. You will build your own vessels, choose your targets for exploration, colonization, scientific endeavor, etc, and blast off for space. The construction system is incredibly robust and fun to use, and, because of the level of customization available, tends to mean that no two ships will ever look or behave exactly alike. There are no food or oxygen or water needs on ships (several mods change this if you want), but the actual mechanics and physics of the game are convincingly accurate, and make for a, while unforgiving, very fun game, because the feedback is extremely consistent. A mistake will kill you EVERY time, unless something hilarious happens (this is actually one of the great parts of playing KSP).
I cannot say enough positive things about this game. I purchased this originally in April of 2013, before it was on steam, and only purchased it on steam now that it has officially released. KSP has few goals, and in sandbox mode, it has none, but goals are not the reason to play KSP. The first time I successfully put a Kerbal in orbit around his home planet, I was dancing around my dorm room, and the first time I landed a Kerbal on the Mun, I felt a sense of accomplishment I have quite literally never felt in a game before. Now, over the course of what must be at least a thousand hours of play, I have built massive colonies on Laythe, the Mun, Minmus, Duna, Eve, and even Ike, I've built huge orbital stations around various planets, and I've even built floating cities in the clouds of Jool and Eve through the use of just a few of the thousands of mods available. I've built resource collection and transport systems including storage, mining, and transport operations. I've explored hundreds of different areas with probes, landers, and vehicles, and yet, to this day, I still play this game at least two to three hours a week, and I'll go on KSP binges several hours long when I think of a new challenge to set for myself. Currently, a friend of mine and I are in a little friendly competition for who can build the most epic Duna colonization project. If you can't tell by now, if you want an 'on rails' game where everything is spoonfed to you, just leave now. This game is most certainly NOT for you. The contract system is honestly only there to give a person inspiration.
But if you want a game that throws challenges at you that will test every brain cell you have, and pound your face into the ground mercilessly if you make even the slightest mistake, then KSP is your game. And when you finally put a Kerbal safely on the Mun, you just might understand why so many people write such glowing things about KSP.