Gareth has complied a lust of all ESP variations that are available from various sources. The sheer number of the different options ESP 01-12 and variations of those implementations amongst the different sellers makes a confusing market place for a beginner. The guide for me is a reference to quickly look-up the differences.
TLDR; version (for me) is to focus on ESP 07 or ESP 12 (for most GPIO pins available)
guide after the jump ESP8266 modules hardware guide – gotta catch 'em all! – layer zero labs.