There are believed to be over 600 Linux distributions, not all of which are active.
because it is operating system
A Linux distribution, known as distro or flavor, is an operating system that uses the Linux Kernel. I think the most common one is Ubuntu.
A Linux Distribution (distro) is lika a flavour. For example: You can go to a dairy bar and ask for ice cream, but you need to specify which kind of ice cream. Chocolate ice cream, strawberry, bubble...