A bob is a common woman's hairstyle in which the hair is cut short, generally no longer than shoulder length, and sometimes much shorter. While a surprising number of styles have been called bobs, the most central aspects is that the hair should be straight or lightly waved/curled, and long enough to reach at least below the ears; any shorter and it risks becoming a pixie cut or a crop.
The classic bob of the 50s and 60s was blow-dried, and curled or waved. Modern bobs often have layers, highlights, and careful face framing -- exactly the same sort of elaborations you might find on any hair-style. Both traditional and modern bobs may include cut bangs.