The above concept art is a bit of a puzzle when you first look at it, what helps is to take notice of the little blue people on the ground, which gives us some scale. So with that help we can gather that these are the huge floating mountains we've heard about, which will tower over the area (and apparently support themselves using some faux vines). As for what exactly the background structures are is unclear, though I think the focus of this piece of art is the flower and plants that will be found in Avatar Land.
Disney also released this video, which shows some of the amazing plants that Imagineers have created for Pandora... the bioluminescent flowers are beautiful. Not to mention the floor that glows as guests will walk over it! If one of the goals of the Avatar project is to keep guests in the park longer this will be a home run - everyone will pack the area once the sun goes down to see this stuff!
Read more details in the original post from Disney Parks.