Men and women are built differently, so their bikes need to be built to accommodate those differences. The distinctions are found mostly in the geometry of the frame and the contact points on the bikes. The differences may not be obvious to the naked eye, but after some time in the saddle, your body will thank you for a bike that fits correctly.
Video of the Day
The Top Tube
Women typically have shorter torsos and arms. So they need a bike that's constructed to reflect that difference. That's why women's bicycles are built with a shorter top tube. This gives women's bikes a shorter distance between the seat and the handlebars and allows a woman to sit on the seat and still be able to comfortably reach the handlebars.
Head and Seat Tube Angles
Women have longer legs than men. So making the top tube shorter on a woman's bike without making other adjustments to the frame can increase overlap of the toe and the front wheel while pedaling. Manufacturers solve that problem by increasing the length and the angle of the head tube at the front of the bike. Doing so brings the front wheel forward more than on a man's bike and extends the wheelbase a little, too, which helps with stability. The seat tube on a woman's bike is at a steeper angle than on a man's bike, another feature that's a result of the shorter top tube.
Your bike needs to have a crank with a length that allows you to pedal comfortably while maintaining your cadence. Women's legs may be longer and their torsos shorter in the proportional make-up of their bodies, but inch for inch, men's legs tend to measure longer. Men's bikes typically have 170 millimeter cranks, but many women's bikes come with shorter ones, such as a 165 millimeter crank, to allow women to get to the bottom of their pedal stroke.
Handlebars, Grips and Brakes
Men are wider at the shoulders than women, so the handlebars are typically wider on a man's bike. They're placed a bit lower than on a woman's bike, too. The grips on the handlebars are different sizes. Women's grips are smaller, to fit better in a woman's smaller hands, and men's grips are bigger, for more comfort and control. In "The Mountain Bike Book", Steve Worland writes that the brake levers on women's bikes tend to have a shorter reach, another point of difference that accommodates a woman's smaller hands.