These are used on longer treks, for distance hiking, or if you're carrying gear for a couple of people, say a child or an inexperienced friend on their first trip. The next thing that separates packs is the type of frame, or if even has a frame. A burlap bag with two strings on it could be forced into being a backpack, but it isn't going to be very comfortable. The strings will dig into your shoulder and the weight of the sack will beat against you as you take each step. The frame allows a suspension to be placed on a pack, which allows the weight to be distributed, and supports your gear.
Frames can be external or internal.
An external frame simply means the frame is on the outside where you can see it. An internal frame means that the frame is integrated into the shell of the pack. Daypacks and some assault packs may not have frames at all. Internal frame packs have become extremely popular. They allow the weight to be carried closer to your body, which helps with your balance, and offer more adjustments.