Disney already owns Pixar, Marvel, and Lucasfilm, and they would only bolster that massive imprint by acquiring 20th Century Fox. That would include all of Fox's film production companies, such as Fox Searchlight and Blue Sky Sutios. It would also include Fox's cable networks such as FX, FXX, and National Geographic (via Mancunion).
It also means Disney might get Fox's 39.14
The big get here, of course, is the film rights, which Disney will get back for its Marvel brand. That means they will acquire the distribution rights for the Fantastic Four as well as the full film rights for the X-Men universe. That would finally allow Disney to integrate the X-Men characters and concepts (like mutants) into the Marvel Cinematic Universe, something fans have expressed interest in seeing for some time.
In regards to the Fantastic Four, the fate of that property is a little murkier than the X-Men. The rights to the Fantastic Four are actually shared by Fox and Constantin, so it remains to be seen if Disney will just outright get all of the rights without some sort of negotiation with Constantin. Disney would still likely get them, as the franchise is anything but hot right now, but it might take some additional discussions.
What is missing in that deal is the core Fox television network. Disney would own the studio, but not the network, so shows like The Gifted and Family Guy could make their way to Disney as well since Fox produces those shows directly.
Final details will be made clear when (or if) the deal is announced, but regardless this deal will change the landscape of superhero films in a big way. It means Marvel fans might finally get a complete cinematic universe, but it will also mean less competition amongst studios in the superhero genre.