The key difference between a bookbag and a backpack is the way in which we carry them. Bookbags are traditionally used by primary school age children and carried by two handles, much like a handbag. A backpack however is carried on the back.
Although they are different, the two terms are still used synonymously. This is especially the case in high school and college where backpacks are used to carry heavy books and learning materials. In recent years, manufacturers have designed backpacks with bookbags in mind. Water resistant rain cover and tough materials, as well as rectangular bottoms and reinforced straps for carrying a heavy load, are frequently used.