You're trapped in a high-tech facility with a psychopathic robot. But hey, she said there will be cake.
First of all, Portal is not just your typical FPS game. Since you don't have weapons properly said, you can forget the idea of running like crazy and shooting monstrous enemies. This game is all about a new way of thinking. You are a prisoner of a facility called "Aperture Science" and are trapped in a series of test chambers, where you'll face several different challenges. The principle is pretty much "there is an exit, find a way to open the door". To help you solve the puzzles, you will have something called "portal gun", which allows you to create two portals in any white surface. The use of it is unlimited and you don't need to recharge it, but you have to use them wisely, either to solve the problems or to save your own life.
There is not much to be said about the gameplay itself. Those who have already played any of the Half Life games might feel pretty comfortable with this one game. The controls follow the same pattern of every FPS game.
The Orange Box version of this game for either Playstation 3 or XBox360 may cause a little problem for those who aren't used to play it on the TV: if I remember correctly, it's a matter of "field of vision", which may cause headache and nausea. This can be set on the PC game, but not on the consoles, and may be rather troublesome. I didn't have this problem with the PC version of the game, but people who are interested in this game may want to think twice when choosing the version of the game.
Overall, this game is brilliant. The puzzles are awfully intelligent and there is an interesting sense of humor in the robot who controls the facility. It's definitely worth checking out this game, even if FPS games are not really your area.