External frame packs are easily recognizable by the metal or plastic frame on the outside of the pack. The shoulder and hip straps usually attach directly to this frame. The main compartment of the pack is also attached to the frame.

Internal frame packs have the suspension sewn into the main body of the pack. Instead of a frame on the outside of the pack, internal frame packs use "stays" for support. Stays are metal or high-tech plastic rods that are sewn into the pack directly behind the shoulder straps. Some internal frame packs also use sheets of hard plastic to give additional back support. Most internal frame packs are one large compartment with an opening at the top. Some packs do have zippers near the bottom of the compartment or along the side to easier access.

Now you may be asking, that is fine and dandy, but which is better? If you ask this to a backpacker the answer will often be "it depends". External frames are noted to be easier to pack your equipment in because they usually have more pockets and it is easier to tie things on the outside. An internal frame pack is known to be far more comfortable than an external frame. Where the external frame usually only touches your back in a few places, the internal frame is made to fit straight on your back, more evenly distributing the weight.

Size is the next big issue. The size, or capacity, of the pack you need depends on what you will be doing. Packs are measured in cubic inches (in the United States) and usually range from 3,000 to 7,000 cubic inches. Large daypacks are usually around 3,000 and might be used for overnight camping if you don't plan on taking much. A good weekend or long weekend (3-5 days) pack is in the 4-5,000 range. If you really wanted to you could take one of these packs on a longer trip, but it would be a tight fit. If you are going out for a long time, over a week with lots of equipment, you might think about a pack around 5-6,000. That will give you plenty of room to pack what you need and not worry about space. Any pack over 7,000 is generally made for long, winter expeditions, or people who feel like they should take everything the own with them on their trip. Internal frame packs also need to be sized based on your waist size and torso length. Many manufacturers of internal frame packs make different size hip belts. Your hip belt should be long enough to curve around your hips, but not long enough to touch in the front. When you buckle your hip strap, you should have a few inches between your buckle and the end of the hip belt on both sides.

Your torso length will determine the length/size of the pack you get. To measure your torso length you either will need a friend to help you or you will need to be very flexible. The find the bottom of your, put your hands on top of your hips with your thumbs facing each other on your back. The bottom of your torso is the imaginary line between your thumbs. The top of your torso is the C7 vertebra in your neck. You can find the C7 by leaning your head forward, then running your hand down the back of your neck. The first big bump you come to will be the C7 vertebra. Your torso length will be the distance between the imaginary line between your thumbs and the C7. Using a flexible measuring tape can help out a lot.

Many new packs are made to be lighter and appeal to the ultralight crowd. The big difference in ultralight packs is in the fabrics. The fabrics tend to be physically lighter, and consequently more expensive. The suspension also is reduced. Many ultralight packs are rucksacks, meaning they have no frame, internal or external. Some ultralight packs will reduce the number of pockets and may even opt not to have a top lid.

With many people using hydration bladders (technical term for a CamelBak), a lot of internal frame packs now have a built in pocket to accommodate these bladders. This is a nice feature, because otherwise, you have to get creative on how to pack your bladder and still be able to access it during the day. (thebackpacker.com)