Backpacks designed for trips longer that a couple of days are made with a metal frame that supports the pack-bag. This frame keeps the weight of the pack on your hips making the backpack easier to carry. Manufacturers offer two types of frame backpacks: internal frame backpacks and external frame backpacks. Each type has its advantages and weaknesses and performs better in some aspects than in others. Today, the majority of the packs available on the market are internal frames, but external frames still have its uses and enthusiasts.

External Frame Backpacks: In this design the pack-back connects to a rigid frame made of Aluminum tubing.

External frame packs are in general cheaper than internal. They are easier to pack. And they are cooler, because they don't huge the body of the hiker allowing the air to circulate.

On the other hand, the main problem of this type of backpack is that it carries the weight high up so you have a higher center of gravity, disrupting your balance and turning your hike into a more uncomfortable experience. This is very noticeable when you want to walk off-trail. Attempting to climb rocks or jump across a boulder field while wearing an external frame could be hard. Besides, they are more difficult to transport when you are not hiking.

Internal Frame Backpacks: This design places the support structure of the pack, typically two aluminum flat bars or "stays," inside the pack. They tend to be more popular nowadays for various reasons.

First, balance: they hold the weight of the pack closer to your body, giving you a better balance during your hike. This is especially important for example when you are trying to cross a creek or jumping over a fallen log. Second, maneuverability: because they are narrower, you can swing your arms freely during your walk, something that is excellent for climbers, skiers, or for bushwhacking when hiking off-trail. Third, flexibility: because they are not rigid, it is easier to transport them when not hiking.

Of course there are some disadvantages too. In general they are more expensive than externals. Once packed it is difficult to grab items from the pack quickly because they only have a large compartment. And lastly, you will sweat more wearing an internal frame, for the reason that the pack sits on your back without room for ventilation.

So, if you like to hike on easy to moderate terrain with a heavy load on your back or you are searching for a bargain, then an external frame backpack could be a good choice. Not a choice that I recommend, but I understand the reasons for doing it. On the other hand, if comfort and freedom of movement are your main concerns or you practice outdoor sports, then stick with an internal frame pack.