A seal of a tailor's workshop
Guilds were a form of organization in the feudal ages, which brought people with the same mastery under one banner. Typically guilds were economic and they were responsible for controlling trade and crafts. Merchant guilds had extensive networks, while crafting guilds produced quality wares for high sums, such as potteries, shoemakers, smithies, etc. The engineer's guild resembles such an array of engineers, who can make and operate wooden mechanical equipment for the army, in return for huge amounts of gold.
Guilds were individual companies and they carried their own symbols to display their field in front of their workshops. Guilds also had a strict role system, with the guildmaster being on the top and the apprentice on the bottom. Masters were often owners of the guild and thus controlled business, while they also supervised the workers and their jobs.
A locksmith's workplace
Apprentices had dire circumstances and had to work hard to earn their keeps. They often were taught only the basic skills, as the guildmaster heavily guarded their ways' secrets from slipping away.
Guilds were the most prevalent form of employment besides the military in the Middle Ages, however their cost-efficiency was inferior as production was very slow. They were succeeded by manufactories: factories which employ more workers to mass-produce wares, sacrificing a lot of quality in exchange for greater and faster production.
As of the Modern Age, guilds remain well in existance to preserve and practice masteries at ceremonial assemblies (and a few other purposes, as well).