It's through here. Extract:
"Portal 2 is everything you want it to be and a little bit more. Which, actually, is probably exactly what you want it to be. Yes, it will warp your understanding of how space and geometry works, but it will make you giggle while that happens.
"If you're one of the two people I need to explain Portal 2 to: it's the follow-up to Valve's 2007 physics-based puzzler that was named, 100% appropriately, Portal. In both games, you're Chell, a test subject made to jump through an increasingly brain-scrambling array of hoops that also double up as holes in the fabric of space. Testing out a portal gun under the watchful (and double-demonic) eye of AI GLaDOS, you have to navigate various hazards and obstacles to get through test chambers and stay alive."
Here, again, is that link.