I've only been using it a few months, but here's what I've learned...
Unix is an operating system, doing many of the same things as Windows, such as mulitasking.
It's multi-user capable, allowing many people to use one computer running Unix at the same time without having to buy a ton of extra ram or the newest processor.
(At work we had many people at once working on a computer running Unix that had only a 266 MHz processor and it was still smooth and stable.)
However, it's command-line driven, meaning everything you want to do is done by typing on a command line, making it harder to grasp than windows where you just point-and-click to get things done.
As for it's uses, people use it for a lot of things, from running an entire website to storing information (such as databases) which multiple (even hundreds of) people can access at the same time. It has more options for security and tends to be a lot more stable than Windows (a friend had his go a year without rebooting... until the power went out
).
Read Chapter 2. What is Unix? for a short introduction
I'm sure you can find a lot more than this all over the Internet.