Basically, "Firefox" is the stable release. It's the one most people use, and should use. It's currently at version 17, for people who actually still care about version numbers :)
On top of that there are three release channels that offer preview builds of varying levels:
* Beta -- is exactly what it sounds like. Beta builds of what will become the next release of Firefox. Beta builds currently are version numbered 18.
* Aurora -- stable but pre-beta. Is two "versions" ahead of mainline Firefox, so Aurora builds are currently version numbered 19.
* Nightly -- the bleeding edge. Rebuilt every single night from latest code, these are three "versions" ahead of mainline, so currently carry version number 20.
mozilla-central is where stuff is true bleeding edge/may kill your cat more often than not, etc.
Beyond saving half of your disk space this is nice because WebM has limited toolchain support so anyone with a video production pipeline basically had to decide whether it was worth the effort integrating a bunch of new tools or continuing to put up with Flash.
Even on youtube, 50% of the videos I try to see cannot be watched without flash.