Big Wigs is a boss encounter add-on. It consists of many individual encounter scripts, or boss modules; mini add-ons that are designed to trigger alert messages, timer bars, sounds, and so forth, for one specific raid encounter.
Snelf's edition is improved version of BigWigs for WoW 1.12 private realms that is actively being developed.
Emerald Dream version is deprecated now. It was based on Mesmerize's modified BW for Freenix/Emerald Dream.
r18321 is original BigWigs from retail (it was pretty basic at the time).