Backpacks fall into three basic categories:
Internal frame packs
External frame packs
These packs are used for single-day hikes, climbs, runs or bike rides. In general, daypacks are soft-backed or frameless. Daypacks are lightweight and intended for light loads (10 to 15 pounds). Good daypacks have hipbelts to prevent the load from thumping on your back with each stride.
Internal Frame Packs These packs are used for bigger, heavier loads (15 pounds and up). Frames--either aluminum stays, plastic framesheets, curved Delrin rods, or combinations of those things--are located within the packbag (as opposed to external frames; see below), and when properly fit, they hug the contours of your back, thereby cinching the load in close to your spine.
The main job of the frame is to facilitate weight transfer to the hip area, which is where we humans are most capable of bearing it. So a good, supportive hipbelt is also critical.
Because internal frames are generally narrower and closer fitting (than externals), they're the best choice for any sort of dynamic activities like climbing, skiing, or bushwhacking, where you need good arm clearance and a tight center of balance. If you typically hike in hot weather, look for an internal with a "trampoline style" back, which means that breathable mesh is suspended across the frame to allow air circulation without any major loss of stability.
Proper loading of an internal frame pack is key, not only in order to keep the weight well balanced and stable, but also to keep you well organized.
Backpacker Tip: Loading an Internal Frame Backpack
Pop your sleeping bag (packed in a waterproof stuffsack or sturdy garbage bag) crosswise in the bottom of the pack. You won't need it until the end of the day and it provides a nice, stable base for your pack.
Next, load heavy items like your food bag, tent (poles can be removed and strapped to the side of the pack), and your copy of War and Peace. Keeping heavy objects low and close to spine will help you maintain the best balance on the trail.
Stuff your puffy jacket and raingear down the sides of the pack, taking up the space left by the bulkier items. (Keep the rest of your clothes in a small stuffsack, and load that in next.)
Use the top lid and other external pockets to stash items that you'll use during the day: snacks, maps, sunscreen, headlamp, and water treatment.