It's an operating system, like Windows.
However, unlike Windows, it's free.
It doesn't operate quite like Windows, although it can be made to. Linux is very basic in and of itself, and to be usable you need to know how to program with it. You can download or buy pre-programmed "shells" for Linux (like RedHat, Ubuntu, etc...) that make it look and operate differently.
It's a lot more safe and secure to use because most viruses and malware don't target Linux users, they go for Windows users because most people use Windows.
Linux is also safer because it can be modified by the user, as explained above. Even with somebody else's version of it, it can be modified.
A lot of programs simply don't run on Linux. Especially older ones, and mostly games.
Hardcore computer gaming freaks like to use Linux because it makes them look hardcore to other computer gaming freaks, even if they don't know how to program.
Linux is great, sure, but, you hear so much about it because of the gamers who talk it up, but don't actually know anything about it. It's sort of like how a hipster talks up an indie band with a stupid name that nobody's heard of; These types of geeks talk up Linux just because not "everybody" uses it.