The primary difference is in the harness and hipbelt and at times colour. The harnesses on the Osprey Women’s Hiking and Backpacking Backpacks, come in torso lengths that best fit women’s bodies. Women’s harness is narrow at the entry point in the backpanel and tapers slightly outward below the collarbone to offer more comfort and freedom for the chest area. The women’s hipbelt is canted slightly upward to best cup the hips and is patterned with a narrower profile.
Colour is another thing that may set a women's hiking backpack apart from a men's. Some companies manufature their women's backpack range in prettier colours, and may use colours such as pinks, purples, teals, or a standard colour with pretty highlights.
But that does not mean that a women can’t use and be comfortable a “men’s” backpack. It really does come down to fit. Use the measuring guides and select the pack that will fit you best.