Everything started thanks to Linus Torvalds, now we have dozens of distributions.
The "Linux" therm generally refers to all the operative systems based on the linux kernel; The kernel was developed by Linux Torvalds as a response to the payed Unix system, this is one of the reasons why Linux is free; Another reason is that Linux is open source and editable.
When Linus developed the Linux kernel it released it under the GNU free license, from that moment the Linux world has started.
Today when we think about Linux we think about a fully working operative system, like Ubuntu, Mint etc. but in reality thoser are distributions that have as root the Linux kernel.
Nowadays there are planty of distributions thanks to the free linux kernel.