Not going to go into detail, but...
Netscape
Corporate whore browser, owned by AOL.. in short: Mozilla with added bloat.
Mozilla
Open sourced application suite comprising of Mail, IRC, HTML Composer and of course, a web browser.
Firebird
Just a browser, based off the browser component of the Mozilla suite, firebird is a slim-line version, more optimised and more user-friendly package.
Thunderbird
As firebird, but with mail.
Camino
Sort of firebird for the mac, was around before firebird was even an idea. Camino is a mac only browser (and just for reference, it's fast as hell
)
K-Meleon
Doesn't exactly fit in, developed by a completely different team, K-Melon has the loose connection of using the Mozilla rendering engine (Named Gecko) to render pages and is basically a slimmed down browser-only package.
There are hundreds of differences between all the variants out there, but that's just the basic stuff, and the only way to find out which one you like is to give them all a try