The appearance of the mesh edges are determined to be evened out or well defined within the 3D Viewport and render. In Edit Mode, individual faces can be selected to determine which faces are smoothed or flattened.
Both Shade Smooth and Flat are also available in Object Mode and function the same way.
Using interpolated vertex normals, the mesh faces will blur at the edges and appear smooth.
Face normals are displayed evenly, because of this all the edges of the selected mesh will be easily visible.