rpm vs. yum vs. apt-get vs. dpkg

[Eric Dunbar]

I've noticed that apt-get is available on my YDL 3 install (I ran it
when I was trying to figure out why "ditto" wasn't available... (I use
it under OS X)).

So what is/are the relationships between these different package &
dependency managers?

rpm = redhat package manager, yum = yellowdog update manager (& now
adopted by fedora IIRC), dpkg, apt-get are related too (does apt-get
use dpkg)?

Anyway, what's the family tree here or is there one?

Eric.