The skybox allow to do quick backgrounds by wrapping an image / generative layers on the background sphere. 
                      By default it uses a SkyMap that can be changed to : 
                      -Cube Map
                      -Cylinder Map
                      -Sphere Map 
                      -Uniform Map
                      -Geometry Mask
                    
 
                    
                    
                      Majority of the SkyBox presets uses Cylindrical maps as input.