It is a kernel, not an os. Anyways, what the f*ck is a kernel. The kernel is the thing that manages the ressources of the system and makes the software be able to communicate with the software. The linux kernel is, guess what. A kernel ! It's a kernel but it's also free and   open source. Why does it matter ? Since it's open source, people can :

1. Make their own OSes with it
2. contribute to it

That makes it one of the most secure kernels. It's also used in a lot of servers since it's also very stable.
There are a lot of OSes using Linux, they're called Distros™. Which brings to :

this is a test

this is a test