A guild is an association of persons of the same trade, interests, or pursuits. You can think of it as a type of club. What that means is a group of like-minded people have decided to form a club or guild in order to communicate ideas, play games, socialize or whatever they wish. In NWN, a guild can be game-related like a Thieves Guild, or can be based in reality like The Old Timers Guild, or technical like a Guild of NWN Scripters. Just about anything can be basis of a guild. Some guilds have over a 1000 members others have 5; it really depends on what sort of guild you want to create or join.
See Category:BioWare Guilds for a list of BioWare guilds.