Thieves belong to Thieves' Guilds. They have little choice – the guilds are much better at finding thieves than the watch, and insist that all active criminals take advantage of the benefits of membership.
If a city has more than one guild, both normally insist that every thief join them, and not the other. This leads to gang warfare, so in a few cities two guilds ignore each others' existence. Thieves in the city must belong to both, but pretend not to know about the other guild. The masters of one guild are low-ranking members of the other, and are generally left alone. Thieves have to pay two sets of dues, and may be given tasks by both guilds at once, making it difficult for them to avoid offending someone.
In a few cases the guild has become rich through respectable businesses, and has a great deal of influence with the legitimate government of the town. Indeed, a handful of towns are actually run by the thieves' guild, in which case the watch is only concerned with unlicensed criminality. Normally, such towns collapse into anarchy, as the guild allows too much crime, but in a few cases the leadership impose stability and a reasonable level of lawbreaking. This often leads to a splinter guild forming, to protect the rights of dishonest thieving folk.
Travelling thieves have to come to an arrangement with every guild they meet. Sometimes this is easy, if the guild requires nothing but a percentage. Others, the majority, impose significant ordeals on potential members, and taking one of those trials every couple of weeks is bad for the health. Travelling thieves thus often try to avoid the notice of guilds as well as the watch.
Warhammer Fantasy is a high fantasy fictional universe created by Games Workshop and used in many of its games, including the table top wargame Warhammer Fantasy Battle, the Warhammer Fantasy Roleplay (WFRP) pen-and-paper role-playing game, and the MMORPG Warhammer Online: Age of Reckoning.
You will learn about all the warhammer tame line and universes.