So far three editions have appeared, each with 'extras' like CDs, transfers, etc. Edition #2 comes with 3 alternative covers (take yer pick, or get completist about it).
No point in explaining, if you don't know Alan Moore, best just to direct you to Wikipedia, I guess.
Or maybe his entry at the Comic Book Database.
And of course, our own previous entries - Alan's wonderful appearance at RAW's great sending off...introduced by the late, lamented and much missed Ken Campbell. Back in Sept 2007.
And how are you tonight, Mr Wilson? from Alan Moore