In a three-quarter view of a face, approximately three-quarters are visible and one quarter is not visible. There is a near eye and a far eye; likewise there is a near cheek and a far cheek, etc. The far ear is not visible.
Three-quarter view is very popular in drawing, painting and photography because it shows both frontal and profile properties and looks much more natural than perfect frontal or profile views. The principle of three-quarter view is used in other areas as well: for example, in advertisement, automobiles are very often photographed in this view.
Many artists initially find three-quarter views difficult to draw. But once they have mastered them, they can draw any view they like. See 3D Figure Drawing Part 3 for a beginner's tutorial on how to draw a head in three-quarter view.