Portal is a puzzle game built on the Source engine, and sharing a lot of the look and feel of the Half-Life 2 games. It's a bit short of play time and content to be a true standalone game, but what it has is so perfect it outshines many full featured games.
Basically what Portal boils down to is a puzzle game with a backstory and sense of humor. It's probably best not to give too much away, as the humor and fun of the game depend on experiencing it with a mostly open mind.
All of that translates to a pretty brief, pretty useless review, but if you weren't planning on play Portal, then this will be the most useful review you've read in a while: Play it. Seriously. It's a quick game to get through, but not nearly as quick as it initially seems. It's funny, it's fun, it's got great puzzles that make you think and then make you feel good about the answer when you solve them.