"Linux" is a free open source operating system. Think MacOSX vs Windows. Linux operates more like OSX, but with much more freedom.
What is free?
https://www.gnu.org/philosophy/free-sw.html
What is Linux best used for? That is certainly an subjective question. 99% of all super computers run Linux. Android runs on Linux. Most consumer routers are running Linux.
Linux has many technical advantages over other OS's; too many to list. Probably the best feature is that is very customizable; infinitely customizable; you can change
anything on your system.
So should you use it on your desktop? The biggest downside to using Linux is a lack of proprietary software that is only available on Windows. Many games will not run on Linux for example. If games or other software that is Windows only is a deal breaker, then stick to windows.