The difference usually would be anything dealing with the absorbency, the location of the absorbent material, the amount of SAP (Super Absorbent Polymer), the number of layers etc. Boys' typically have most of the absorbent material concentrated in the front, while girls' have it in the middle/bottom. Also the fit of the garment would differ, in that boys would have more room in the front area, while girls' would be cut differently and accommodate larger hips, while boys would not accommodate girls hips especially those of an adult and just not really fit properly or cover all the appropriate areas as well.
Basically boys specific products usually are designed to fit and work best with boys 12 and under roughly. Girl specific products may not accommodate the waist or large thighs of an older girl or adult but the absorbent area would be in the right area for a woman. For a girl, to wear a product designed for a younger boy the product would have all the absorbent material in the wrong area, that would be of no use to a girl, and due to the cut and sizing it would not fit correctly or cover your butt most likely.
If you are actually speaking about a product for an adult male compared to an adult female then that is a different story, if that is the case, let us know and we can explain those differences instead.