Guilds are different from factions in that they are professionally aligned rather than ideologically or politically aligned. It is possible to be a member of both a guild and a faction. A guild is a sworn & professional association, a drinking club, a charitable society, and an economic agency.
On Valyr guilds were most common in the cities from the sixth century and later. The older guilds were generally more limited in character, being primarily religious or social in nature. In the towns of the central Bansu Castle area, however, they are more organized around a trade and this is how they appear in their most common form today.