The 2e Complete Book of Thieves was one of the best books to come out of the 2e era precisely because it answers this question so well.
Picking pockets is just one way to work the streets. A cut purse is a thief that uses a small blade to cut open purses or purse strings. A hooker is a thief that uses a pole with a hook of some sort to pull things through open windows. Professional thieves will often work in teams with one thief providing a distraction, one thief doing the work, and one as a handoff man who specializes in getting the loot out of the area. Often their will be a lookout as well, who often are also professional beggers of some sort. Actual theft will probably be a lower percentage of the guild revenue though than cons. The advantage of a con job over theft is that the victim is usually embarassed rather than angry. Professional beggars such as climesmen (who fake injuries, infections and diseases) make up the lowest rungs of the guild. Other cons run rigged games of dice or cups. More skilled cons use a variaty of complex swindles, usually by luring greedy people into doing something illegal and then pulling some sort of switch at the last moment. When a con can't find a mark, he'll often work as a tout - a guide for visitors to the city. The tout tries to steer visitors to guild owned establishments like brothels, drug dens, gambling halls or fences. He'll also propose to smooth the visit by paying various bribes to officials to avoid taxes (which may or may not be real). Crooked officials may work with the tout. An angry tout who feels that the visitor is being disrepectful may try to steer the visitor into trouble - such a pick pockets or muggers - or trick them into breaking the law.
Big complex jobs call for top story men, who enter houses through upper floors - what we'd now call a cat burgler - and box men that open safes and strong boxes. The guild is also likely to have a forger, who creates false documents and papers for the guild. In edition to fake documents, forging may include fake religious trinkets, fake antiques, fake magical items, fake medicines, and low quality goods made to look like higher quality ones.
One of the most important individuals in the guild is the fence, who buys, legitimizes, and sells stolen property. He will have contacts with crooked merchants in the city and with fences in other guilds. This makes him also critical in the smuggling industry, of which he is likely the defacto head. Hot property will be smuggled out of the city to sell elsewhere, sold to crooked merchants who provide legitimate fronts, or taken to crooked craftsman who will tear the item down for its valuable parts. Many of these won't be a part of the guild, but just business partners. Smugglers may be in the guild, or they may just as likely be independent 'contractors' that work with several guilds. Smuggled goods will be anything that has tarriffs and taxes, which could be something like tea or coffee, stolen goods, and illegal substances like narcotics, poisons, dark magical items, the paraphenial of forbidden gods. In a port city, the fence likely also has contacts with pirates, the captains of which are defacto heads of their own guilds. The fence himself will attempt to have a very honest front.
It's likely that the guild will have in it or be associated with a crooked apothecary for when the guild requires poisons - often for drugging gaurd dogs. In D&D, the guild will also likely contain or be associated with a crooked hedge wizard that provides the guild spells, and one or more clerics of some diety that looks favorably on theft. In D&D, the cleric may well be at least the nominal head of the guild, and responcible for officiating during formal meetings, mediating conflict between or passing judgment on guild members, and training new thieves. In my game, one rather common background for wizards is a 'gutter mage', who learned his arcane arts from a lineage of wizards who practice deception. These are either in guilds, or sometimes - on the theory you never interfere with a wizard - independent contractors who travel between cities plying their trade and helping guilds with enchantments and spells.
New recruits to the guild often enter into the ranks through a kidsman - a person that recruits orphaned or abused children to act as thieves. Older girls are then generally traded to pimps or madams become prostitutes. Likely boys enter the thieves guild. Those of no use might be trafficed as slaves.
Any sizable guild will have a number of enforcers who protect guild interests from rival gangs, criminals whether in or out of the guild that refuse to deal with or try to cheat the guild, and overly zealous good doers. These in D&D will be multiclassed fighter-thieves who are skilled in combat. At times the enforcers will recruit muscle - imporsing brutes sympathetic to the guild but perhaps not actually in it - to help in their enforcement. In your city, these are likely crooked stevedoers and teamsters that help with smuggling. The enforcers also likely run the guilds protection rackets, and act as bouncers in the bars, brothels, and gambling halls. Smart enforcers are also likely to end up running the guild, are they may end up in a tension with other guild leaders like the fences or clerics resulting in a sort of oligarchy rather than single leader. Alternately, leadership of the guild may have become concentrated in the hands of a wealthy or even noble family, so that it is heriditary. In D&D, the leader might possibly be a powerful magical being of some sort.
Gambling operations are important. They'll include not only games of chance, but betting on violent animal contests such as 'ratting' (pitting terriers against bins of hungry rats, you bet on how many rats they'll kill in a certain time), bull baiting (whether a dog can pull a bull to the ground in a certain time), dog fights, cock fights, and pit fighting or boxing between humans. Fighters who work in the pits often also serve as expedient muscle for the guild, as for that matter can particularly vicious dogs and their handlers. Bars and brothels will often be associated with gambling operations and drug dens. They'll often employ entertainers and musicians, serving staff (drunk patrons are good patrons), and cooks or bartenders. Young boys called 'fancy boys' will sing and dance to amuse the guests, help with seving, and sometimes serve as catamites depending on the tastes of the patrons.
As might be somewhat obvious, a guild of say 50 individuals might have many many times that many crooked associates that work with the guild in some capacity or who are dependents of the guild (wives and children, slaves and servants). The guild (or gang) forms much of the aristocracy of the cities criminal element, but they are not the whole of it. The smaller the guild, the more hats each member will have to wear. The larger, the more time that they can devote to a single pursuit. Fifty theives and similar criminals might be running an economy that supports 500 assorted commoners, experts, fighters, and aristocrats and which, through its legitimate fronts, provides considerable tax revenue to a port city (partially offsetting the loses from smuggling and the like). Weary law makers may often give up at beating the guild by enforcing anti-smuggling laws, and instead move the tax and enforcement burden to areas that they can control, unofficially tolerating the guild or even partially legalizing their activities as long as they don't draw attention to themselves or inconvience that law abiding part of the society. If the government is particularly corrupt, the guild might actually gain a reputation of being the more trust worthy and effective portion of society (imagine a LE guild in a CE society), and so otherwise law abiding individuals will turn to the guild for justice, recompence, protection and fixing legal troubles.
It's generally unlikely that the guild will pick trouble with 'slayers' like typical PC party members. They'll also typically avoid being confrontational if caught. It makes a lot more sense to try to patch up relations with powerful figures than to get into showdowns with them. Violence becomes an option only if diplomatic efforts fail and the PC's go after guild members, particularly those not directly connected with the original crime, or if the PC's cause the guild to lose face.
) business. could be a market or a butcher...inside the shop are all sorts of doorways and secret rooms to the thieve's guild proper. proprietor of the business could be a conspirator or even the head of the guild.
