The Thieves Guild is a criminal organization consisting of rogues and thieves found all over Tamriel. It is based in the city of Abah's Landing, found in Hew's Bane on the southern coast of Hammerfell. The guild is the subject of the Thieves Guild DLC. Joining the guild or opening your first Treasure Trove provides access to an exclusive skill line. Like with the Fighters and Mages Guilds, progress through the Thieves Guild skill line is achieved by gaining reputation. This can be done by completing faction-related DLC content. Losing access to the DLC will disable this progression, but all earned skills will still be unlockable and usable. There are 12 ranks of reputation within the guild.
Each guild have their benefits, it can be usefull skills for defeating enemies, like Fighter and Mages guild can give you, or skills, that can rise your wealth in world of Tamriel, like Thieves and Dark Brotherhood. Also for great adventurer, that like to explore - Psijic guild offers the best skills for that and provide some extra option to find mysterious Portals with lots of spicy rewards.
All of this content is waiting for you, but to reach it out, you will have to lay your path throw a hundred of hours time-wasting repetative grind and quests, that most of the time foul the impression of the game. But don't hurry up to scream, that it takes "FOREVER!" to reach your desired point! Because we are here to help you out! No more lazy rats killing to get needed rank to level up, no more quest skipping to became champion!