Women’s sleeping bags are typically designed with a slightly different shape and insulation layout to better suit the average female body and sleeping preferences. They are usually a little shorter than a unisex regular-length bag, narrower in the shoulders and roomier in the hips.
Many women's bags also include additional insulation, often in key areas like the chest and footbox (possibly in other areas depending on the bag). The amount and placement of this insulation can vary depending on style/design, so there is not a hard and fast rule for how much extra insulation a women's bag receives.
Additionally, temperature ratings are often listed differently. Usually women's bags are generally rated for comfort temperature (the temperature at which the average sleeper stays comfortable), while the unisex or men's bags often use the lower limit or survival rating, since women tend to get colder than men.