I think this page covers it better than I ever could.
https://simple.m.wikipedia.org/wiki/Linux
In a way, you can think of it as an OS analogous to Windows Phone for the PC market. Its relatively rare on personal PCs. Its more of a generic term though since there's a lot of Linux flavors, or "distributions", like how there's many automotive brands, yet they're all cars.
Many OSes have Linux at its core like Android.
Reasons you may want to try it are that you want something different than Windows and Mac OS. Or you are uncomfortable with the proprietary and potential privacy issues with Windows. Or you want something more secure both in nature and its relative obscureness. (like WP) Or you like a lot of freedom. Or you have a computer where a typical Windows install may be a bit too heavy.
The chief downsides are that it may not support all the hardware 100% depending on your specific configuration, it sometimes requires getting into the Command Line/Terminal more than its main competitors although its become increasingly user friendly in recent years. Mainly though, like WP is that it doesn't have all the programs available, since most are written for Windows. However you do get options like Virtual Machines and some fancy software that can bridge the gap as well as a variety of alternative programs and Linux versions of those programs like Firefox, etc.
A popular Linux distro is Ubuntu, and is a good place to start.
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