DLC has a lot of content. Of course, the main part of it is a widely know to TES fans Thieves Guild. The update adds Powerful new gear, unique crafting styles, and other rewards inspired by the iconic Thieves Guild. The patch notes are heavy and filled with changes and content. It is hard to describe them all. So, here is a link with the original release.
Here is what Creative Director Rich Lambert thinks about the content:
"Welcome to the Thieves Guild! All of us here couldn't be more excited about what's in store for you with this latest DLC game pack. The zone of Hew's Bane is gorgeous, with some of the most spectacular world building to date and equally compelling content. There are new types of traps, passive skills, delves, group bosses, and quests that take you all throughout Tamriel. There are also all new justice activities which the Thieves Guild rewards you for completing! If you love stealthy gameplay and being a thief, you are going to love this DLC."
Lastly, there is a trailer devoted to the lunch of DLC on consoles: