Long-haul backpacks, or frame packs, are made for two days trips or longer. They use a metal or graphite frame that supports the pack on your hips. There are two styles to this, the Internal and External frame. Internals are narrower and have a tower like appearance.  

They integrate their framework inside the pack. The "stays", or flat bars are usually aluminum and are configured in a V-shaped. Composite stays are usually parallel, X-shaped or U-shaped.

Stays are removable and are made to conform. The good points about an Internal are: Flexibility, Balance, Stability, Maneuverability, and Adjustability. They tend to be more comfortable, cushioned, and a lot more flexible then an External. The downside to Internals is: Usually only one main compartment and you will sweat more because the pack is closer to your back than an External. Externals also have their advantages, such as: they are cooler to carry, easier to pack, the pack won’t tend to sag, and the cost is lower for an External. The downsides to an external are the minimal agility and difficult when traveling. The capacity in choosing a pack depends on where you are going and for how long. For a one night hike use a pack no bigger than 3,000 cubic inches. For a 1-2 night trip use a pack between 3,000-4,000 cubic inches. If you plan on going for a 3-4 day hike take a 4,000-5,000 cubic inch pack. For a hike up to 6-7 days take a 5,000-6,000 cubic inch pack. For any trip longer than 7 days, plan on taking a 6,000 plus cubic inch pack.