Six degrees of freedom. Six degrees of freedom. Six degrees of freedom.
Descent isn't much, as far as story is concerned. You're tasked with clearing out a series of offworld mines filled with deadly robots, and...well, that's about the extent of the plot. But Descent is a classic, in large part because it's so unique. It's a first-person shooter played in zero gravity, allowing you to move along the x, y, and z axes, pitch, yaw, and roll (the aforementioned six degrees).
Using a joystick is highly recommended.
Modern perspective: Descent 's not the prettiest game nowadays, but it's still worth playing because there's nothing else like it. Literally. The whole six-degrees-of-freedom thing never really caught on, so good luck finding a modern first-person shooter that plays like Descent. Well, unless the planned sequel attains funding.
If you need something that looks a bit prettier, Descent 3 is also a great game (though less fast-paced than the original).