Women's bikes are fit to the individual rider in exactly the same way as men's bikes: the rider's body measurements dictate frame size, saddle height, handlebar width, stem length, crankarm length and saddle position fore and aft. Women's bikes simply make the fitting process easier because their geometry is already female-friendly. Instead of forcing a woman to fit the bike, the bike now fits the woman. Bike fit is crucial to rider comfort and pedaling efficiency, as well as injury prevention.
Women riders (even performance-oriented cyclists) generally prefer a slightly more upright riding position than men. This is due to the female pelvic structure and has nothing to do with a desire for a less aerodynamic position. Women's saddles help with a female cyclist's comfort on long rides, but raising the handlebars slightly (as compared with a man's bike) takes additional pressure off the soft tissue in the crotch.
Handlebar width is important on performance bikes. The hand position must be neutral, with the hands either straight out from the shoulders or very slightly either to the inside or outside, but only slightly. Most men's road handlebars are too wide for women, but on women's bikes, they are proportionately sized. On mountain bikes and hybrids this is less important. Mountain bike handlebar width is a very individual preference; but women's mountain bike handlebars are narrower than on men's bikes.
Women's bike sizes are generally a few sizes smaller then men's bikes. This range of sizes accommodate riders from about 4'10" to 5'10" in height. Road frames range from 42cm to 57cm, and mountain, hybrid and cruiser frames range from 13" to 18". Taller women can often ride a men's bike, but very short women almost always need a women's bike.