The vast majority of bikes are designed around the proportions of the average male physique. Women are built differently (obviously!) with a shorter torso and proportionally shorter arms and longer legs. Women's hands and feet are smaller, their shoulders are narrower and their hips are wider. All of these differences can affect the way a bike fits, feels, and handles.
Womens specific bikes are designed to these specifications. Most brands also have a separate design and color scheme for womens specific bikes, which is another feature that differentiates womens specific bikes from non-womens specific bikes. Women's specific urban city bikes usually come in a step-through frame option with a wider seat and higher and more narrow handlebar. Womens specific beach cruisers have different curvatures in the upper frame and slimmer tubing.
Road Bikes and Mountain Bikes that are women's specific come in smaller frames sizes, are equipped with shorter stems, shorter handlebars, and most often have a top tube that declines to make it more comfortable for a female rider. In addition to that some companies offer a different frame geometry for women's specific bicycles.
Bike Attack Santa Monica sells a wide variety of women's specific bicycles from Bianchi, KHS, GT, Norco, Scott, Linus, Biria, Virtue, Pivot, Haro, Redline, and Schwinn.