Shade Smooth & Flat
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.
Note
Both Shade Smooth and Flat are also available in Object Mode and function the same way.
Shade Smooth
Reference
Mode:
Edit Mode
Menu:
Face ‣ Shade Smooth
Using interpolated vertex normals, the mesh faces will blur at the edges and appear smooth.
Shade Flat
Reference
Mode:
Edit Mode
Menu:
Face ‣ Shade Flat
Face normals are displayed evenly, because of this all the edges of the selected mesh will be easily visible.
Tip
Use the Edge Split modifier and Smooth by Angle Smooth to balance between smooth surfaces and sharp edges.