Of the three different backpack types, the daypack, is probably the most recognized. Visit any school and most students will be carrying a typical daypack. Compared to external or internal frame backpacks, daypacks do not usually have any frame at all. They are the smallest of the backpack types and the least expensive. Daypacks, as their name suggests, are designed for simple daily tasks or an afternoon hike. They are perfect for carrying books, school supplies, and maybe a small lunch bag. If using a daypack for a hike, you can usually fit a sweater or jacket, a limited amount of gear and enough food for daytrip in good weather. You could probably use a daypack for an overnight trip in a pinch, but it would take some creative packing and cooperative weather. There would be no room for foul weather gear.
External frames are exactly what their name indicates. They are usually large and have a lightweight metal frame surrounding them. Unlike internal frame backpacks that require everything to be carried inside the pack, the external frame pack allows you to attach things to the frame. Although tying things to the frame can save space in the pack, it can also be a disadvantage in certain situations, like climbing. The external frame backpack is designed for the serious hiker and works well on flat and gentle terrain. These types of backpacks are also uncomfortable. They are big and bulky and somewhat awkward, and do not fit well against your body. Although external frames are great for carrying a lot of gear, they just don’t measure up when it comes to comfort.
They say that necessity is the mother of invention. This certainly holds true for the creation of internal frame backpacks. Basically, internal frame backpacks were designed to eliminate all the drawbacks associated with the external frame models. There are a few design features that give this backpack a definite edge over the external frame model. Internal frames are built with the frame on the inside of the pack. This frame is flexible and allows the backpack to fit the contour of your back, resulting in a much more comfortable fit. Internal frame backpacks are also wider at the bottom. This feature lowers the center of gravity making it much easier to balance. You do not have the outer frame to tie things onto, but depending on your needs this can be an advantage or a disadvantage. The fact that everything has to be carried inside internal frame backpacks is a plus when you want to avoid sagging or wet snow. You also do not want anything hanging off your pack when you are doing any off trail hiking or climbing.