The King of Thieves rules over the Guild and supplies members with equipment and direction. He uses his ties within the government to make stealing less risky. In exchange, guild members give around 20% of the money they steal to the King.
The Thieves Guild is an attempt of various thieves to work together to steal more money easily.
Guild members get access to various experts in the field of thievery that have also pledged to the King. For most crimes, Guild members also have protection from arrest due to the King's dealings with government officials. Sometimes, guild members can receive equipment or other goods that have been stolen by other guild members. All of this requires a percentage of income to go to the King.
The Guild was started by the drell who would later become the King of Thieves after reading about previous attempts at collaborative thievery. After finding cooperative enough thieves, he began putting together resources to learn about easy targets and directing the first few members. Eventually, when he gained enough power and wealth in the underground, he was able to find connections in the government, getting nicer sewers built and guards to start looking the other way. Eventually, the drell became known as the King of Thieves and had a near monopoly on thievery.