There are four total kinds of structures that you can build in Fortnite: Walls, Floors, Stairs, and Roofs. They can all be edited to fit into different sizes, styles, and materials. All of the structures can be built with Wood, Stone, or Metal.
After you select the structure you want to build, you can edit the structures using "G" as the default key on PC/Mac or with pushing in the right analog stick on console. Below are some examples of the structures you can build, with the blueprint, and end result.