Limbo is an awesome game, that I just had to write a guide for. If you haven't played it, or bought it, go do so now! Really this game brings something back to gaming that's been missing since the n64. Fun. Okay end shameless plug.
In Limbo you play, I would assume, a dead boy who's trying to make his way through this world.
You can only do a couple things. You can Jump, Grab, Climb, and Swing. Wait. I feel like I'm leaving something out. Oh yeah, you can die.
Well start running. You will eventually run on to a log. Jump or slide off the log to get to the slope below you.
You will end up sliding down the cliff to a pit at the bottom.
Jump over the pit. There's really no need to do this, but it's a safe way to practice for later.