The following NPC Trade Guilds exist
Society of Thieves - Thieves
Warriors' Guild - Warriors, Weaponsmiths, Guards and Paladins
Guild of Arcane Arts - Mages and Alchemists
Society of Clothiers - Tailors and Weavers
Bardic Collegium - Bards
Order of Engineers - Tinkers
Guild of Healers - Healers
Maritime Guild - Mapmapers, Shipwrights, Fishers
Merchants' Guild - Merchants ,Provisioners, Jewelers, Innkeepers, Tavernkeepers
Mining Cooperative - Miners
Blacksmiths' Guild - Blacksmiths and Weaponsmiths
Society of Cooks and Chefs - Cooks
The requirements for entry to any of these guilds (in addition to not being a member of another trade guild) is 48 hours of in game time, and 500 gold paid to the guildmaster. To join a guild, find the appropriate guildmaster and say "<guildmaster name> join". He will then ask you for 500 gold if you meet the qualifications. Just drop the gold on him (the exact amount) to join.
The Thieves guild has an additional requirement of the character being 7 real days old, and have 60 base stealing skill before membership will be considered.
To quit a trade guild, you must spend at least 4 days (96 hours) of real game-play time in the guild, and must remain a member of the guild for a full week.
The primary benefit to joining a trade guild is a 10% discount on their members goods. For additional benefits of the Society of Thieves, see the Stealing skill.