The Oval face is considered to be the ideal shape by hairdressers. People with an oval face shape can wear most styles and lengths with it still looking "right". The oval face is mathematically 1.5 times as long as its width, with the forehead slightly wider than the jaw.
Round faces are characterised by a round chin. Because they lack the length of the oval face, the aim of a hair style should be to create the illusion of length in the face. This is achieved by creating height on top of the head, and minimising hair width -- keeping it flat at the sides and around the ears.
Rectangular faces have a square chin and hairline, and are long and narrow. The aim of a hair style should be to create width at the sides to disguise the narrowness. A fringe can also shorten the face, and a side part will help to disguise the squareness.
Pear shaped faces have a narrow forehead and is widest at the jawline, with a round chin. The objective of a hair style should be to give the illusion of an oval face by creating width at the forehead and temples.