Thieves
Thieves' guilds gather like-minded individuals who make alliances, plan jobs, and get information on buildings, people and security measures. Being part of a thieves' guild provides the same basic benefits as any other guild: insurance, training, and tricks of the trade. Members get access to specialized equipment and information to make their job easier. If a member of the guild gets into legal trouble, the guild may be able to pull some strings and arrange their release, though the member will likely have to reinburse the guild for its trouble.
Another benefit of guild membership is more sophisticated thievery. A guild allows for such things as protection rackets, where people pay the thieves' guild to insure they, their homes and their buildings are not burgled. This only works with implicit cooperation from guild members, of course. If a merchant pays protection money but they are robbed anyway, they are not likely to pay again. This is the main reason why independent thieves are not tolerated within a guild's territory--not only are they not paying the guild its fair share of the loot, but they could irreperably damage the guild's reputation.
Thieves' guilds also build up a repetoire of snitches, informants, bribed officials, and magic connectiosn that other guild members may use. Smuggling goods, either for direct profit or through fencing, is also easier when thieves work together. The officials may catch one or two thieves, but the operation goes on.
Although a strong leader might occasionally unit various factions of thieves into a single guild, it is more common for multiple guilds to stake out territory throughout the city. The Black Hands claim gambling and girl rights in the docks ward, and the Unseen claim pick pocketing and begging on Baker St, for example.