Hello friends, Ok I am a little confused. I downloaded some shp files of the world (countries, rivers, cities...). The coordinate system is GCS_WGS_1984. So is there any projection that i should mention in the layout of the map? Also on my screen i see 2D shapes not a 3D sphere so what means using a Geographic coordinate system in ArcMap? Thank you in advance.
When a data frame in ArcMap is using a geographic coordinate system, an internal projection occurs. It's very simple--the latitude and longitude values are treated as if they're linear and simply displayed. This is sometimes called a pseudo-Plate Carree projection. A true Plate Carree projection would convert the latitude and longitude values to radians and scale them by radius of the earth model in use.
Many people would say that you shouldn't make a map (particularly a printed one) using a geographic coordinate system. Instead you should pick an appropriate projected coordinate system based on the purpose of the map and its extent.