Portal is a charming, atmospheric, light-humoured puzzle-fps with dark undertones and an engaging story. It has also been praised by feminists as a well-marketed game from a major games company which features a believable female (silent hero) protagonist, where the industry default would have been just to stick a man in that role. The game gives you plenty of time to approach each puzzle, but slowly increases the difficulty and teaches you new tricks to solve the later puzzles.
The basic concept of the game is: what if you had a device which could create a join between two surfaces through which you can travel? Now escape this research facility.