It is an alternate Operating System for a PC or server. It is open source which, at least in theory, means that it is free and the user community contributes to it's development by giving their time free.
Now several companies have taken it and added their own little twists. So they sell the OS with their additions (only charging for thier additions). These are normally called 'distros' or distributions.
It is similar to Unix. While it works well and the price is right, you have to do the work and understand the loading of the OS, find your own drivers, find your own software, predetermine drive partitions, understand root access, etc. So it is not for the timid.
If you have a spare computer laying around, doesn't have to be real powerful either, then it is quite educational to play around with. Just be prepared to reload your OS many times, until you get it the way you want it, especially if you are just learning. There are many free versions around so the education is pretty cheap.