Given that readers come to interpret and justify specific aspects of any text in different ways, the best way to describe anything is to find your own understanding of the text itself. In this case, you will be examining how Suzanne Collins gives readers information about the different districts in The Hunger Games.
First, you will want to look at the limited information given on the settings of the districts themselves. Katniss offers some very specific descriptions about the different districts as she travels to the Capitol.
Another way to describe and define the different districts is through Katniss' descriptions of the tributes. Each of the districts offers two tributes to attend the Games. Kantiss provides her own interpretation of the tributes for each district which provides information on both them and the districts which they come from.
In the end, based upon the information in the text, you should be able to define and justify your own interpretation of the districts based upon both physical descriptions, inferred information (information which you decide upon based upon how you interpret the information given), and Katniss' descriptions of the tributes.
As a hint, the best descriptions are going to be those which are not directly stated in the text. Interpretations based upon inferred information will provide a much more rounded and deeper description of the districts.