The Thieves Guild, much like any guild, was created to bring those with a certain set of skills together for a common purpose, and to provide sanctuary for the less savory types in Hyrule. It is often a last ditch effort for survival when one decides to become a thief, and more often than not will happen to perfectly respectable people. As a way to avoid being tossed away by the moderately lengthy arm of the law, they can join the protection of the Thieves Guild as long as they share some of their wealth.
Location
The Thieves Guild HQ is located just outside the west end of Hyrule Castle Town.
Guild Structure
The Thieves Guild is led by the monstrous Viscen "The Bison" Tagron. After taking over the guild, he took the initiative to quickly grow it into what it is tooday, spanning across major cities in Hyrule and even some smaller branches in neighboring countries.
Blahblah leadership, blah, probationary period, blah blah, how they get paid, blah blah, the kinds of jobs they take and how they're oh so much better now that they have a 9' tall douche leading them. IDFK.