This is why I play video games.
Essentially, Portal is a puzzle game. Sure, you can die, but the biggest part of the game is devoted to figuring out the various puzzles presented to you by the omnipresent AI. I was astounded by this, since the engine it is built on, the Source engine, has been responsible for action-fests like Counter-Strike: Source, Half-Life 2, and Team Fortress 2. Don't get me wrong, there are action-packed parts--in fact, the balance between puzzle solving and action in this game is pristine. However, the definition of action is a little different, namely in that it doesn't mean shooting. You never possess a gun, only the "Aperture Science Hand-held Portal Device." In other words, the Portal Gun.
The portal gun in itself could be a game. There's so much fun stuff you can do with it, like creating an infinite loop, or propelling yourself across the level. Many of these fun things are part of the game, making them even more enjoyable.
However, the portal gun isn't what makes this game unique. It's the atmosphere and the humor. This game has the best sense of humor in any game I've ever played, ever. I won't spoil any of it for you.
Why am I still rambling on? Buy the Orange Box, or at the very least, by Portal--you can get it separately on Steam if you want. I recommend the entire Orange Box though, since you get three EXCELLENT games for the price of one. Unless you hate yourself, buy it. If you hate yourself, buy all three games separately so Valve gets more money.