The Guild as it stands today has gone through many transformations, though its origins come from the old tradition of the shipbuilding Castellani family and the fishing Nicolottis. The two warring families created the first guilds in Venice, groups that ostensibly would work together for the benefit of their own workers, but realistically were only created to organise the fighting between the two families. The Guild consists of both of these groups and more, coming from destitution into power.
Venice always had a number of guilds representing the various businesses of the city. As Venice struggled financially the Guild's lost their power and pull over the running of Venice, eventually becoming nothing more than a collection of out of work, impoverished commoners. This lead to them turning towards the criminal underbelly of Venice, in little time the Thieves Guild became the biggest and strongest guild of Venice.