Pack frames are used mostly for backpacking trips. You can carry in a backpack frame all of the essential equipment you need to survive in the wilderness. This is the main reason why this type of backpack is so popular. It is the most important piece of outdoor and camping equipment. Pack frame consists of a visible metal frame that supports a nylon pack.
There are two types of backpacks, internal and external frames. First have their support structure hidden within the pack. External have a metal framework on the outside, i.e. it consists of a visible metal frame that supports a nylon pack. This makes them less expensive. They provide better ventilation in warm climates and they are generally older and less complex in design than internal packs. Also, external pack frames are better for carrying heavy loads on smooth trails. This is because they shift the pack's weight to the appropriate areas of the body.
Internal frame backpacks are much more form-fitting than regular backpacks. They are designed to transfer a substantial portion of the weight to the hips. One of the reasons that makes them so strong and long lasting in spite all the abuse they are exposed to in service, is that they are made out of aluminum or carbon fiber and nylon. They are lightweight, strong, and durable materials with strong tensile. Also, they are not susceptible to corrosion or deterioration in extreme environments, and effects of extreme cold and heat. And, they require no maintenance over their lifetime except occasional washing.
An internal frame backpack fits much better. This means you will stay balanced and in control. The major reason for this is because the load moves when you move and doesn't sway from side to side. They are more practical, and extremely comfortable. With the built-in aluminum or carbon fiber frame, you can carry much more than you can in a rucksack, and will be more comfortable over distance. Comfort, versatility, and balance are the makes them so good.
External frame backpacks cost a bit less than the internal. That is why it is so popular amongst beginners. They are best suited for amateurs, although many professionals use them as well. External pack frame provides the most stable base and are the easiest to handle. The aluminum or carbon fiber frame distributes the weight evenly among your upper body, over the shoulders and the hips to alleviate the spine from too much pressure, making that walk up the hill much easier. They are meant for heavy and long term use.