It is a simple platformer, presented in disturbing black and shades of grey. You are a little boy, and you wake up in the middle of a forest. There is no backstory, no text on the screen, and no loading screens or levels.
The description of the game on Steam and Xbox Live Arcade says nothing except:
"Uncertain of his sister's fate, a boy enters LIMBO"
That's all the backstory you get. Apparently your sister is in this world with you.
This is the most haunting and atmospheric game I have ever played, and the ending is beautiful and shocking. I can't even explain how I feel about this game. The sound, visuals, and gameplay tie together better than any game I have ever seen.
It also won over 90 awards, so I suggest you check it out. It's not often a platformer wins: "Best horror game" Here is a gameplay video with no spoilers:
http://www.youtube.com/watch?v=tZhiqVE0E5w
