*Spoiler Alert! The last 5 minutes of the video contains the first cut-scene of the game.*
The kid that does the walk through is the Norwegian n00b that got the game 2 weeks early. I say n00b because this kid didn't even know about half the features/weapons in the game. Now before you get all critical and say, "But Jayman not everyone is crazy obsessed about Halo 3 like you are, and most people have jobs and lives...etc..." I hear you I really do but, it's not like this kid just happened to be walking through Best Buy and saw a bunch of retail copies of H3 sitting around. He has been very proud of obtaining Halo 3 early and very vague about how he got it.
So he is obviously some what of a Halo fanboy, and anyone that is planning on purchasing H3 has at least heard of things like The Forge before. At any rate, here's a descent look at some of the multiplayer levels.