I find myself trying to explain “Linux” to people interested in Linux. I try to explain to them that Linux is NOT the operating system, but only the kernel underlying the system. I try to tell them there are hundreds of “types” of Linux and generally that is followed promptly with a glazed over look and more questions.
I went out on Google and searched for “Linux Timeline” then clicked Images. I found some great ones, showing how Linux has evolved from Linus Torvalds’ baby, to what it is now.

- Linux Timeline
The above graph I really like because it takes into account many of the distributions left out in the graph below. The detail isn’t there like the lower image, but the distributions are represented a little better.
And another version, I like a little better (Shows a more defined starting point):

Little different timeline
Some things I see on the one above is that Xandros Linux has changed hands or merged with Linspire. Intel has created their own Linux distro, Gentoo has a few spin off distrobutions and Ubuntu also has MuthBuntu now although not “officially” supported by Ubuntu. You’ve got a few things with Mepis as well. They have bounced from Debian proper, to Ubuntu and back to Debian. I think it would be near impossible to actually show EVERY Linux based distrobution out there, but I would love to see that project come to fruition.
I hope this clears up a little confusion. I’m sure there are better explinations out there, but these two seem to get pretty close to nailing the needle in the hay stack =) .
Cheers,
Justin