Linux has really developed and grown since it first introduced to the world back in 1992. Because it's an Open Source project, it has been passed around, improved, upgraded, updated, and renamed (as distributions). Linux is constantly evolving.
If you want to get a a visual look at how Linux has developed, check out the online chart at:
You'll even discover when the Linux distro you use was created/launched, and which main distribution it is based upon.
This chart has been published under the GPL, so you're free to download, print out, and share with others. And