Internal frame packs have the plastic frame inserted in the pack. The pack wraps around your back so that the weight is transferred over a large surface area. The weight is held closer to your body, which makes it easier to handle in rough terrain.
Internal frame packs are very popular nowadays, and account for most of the packs sold.
External frame packs have a rigid frame which the pack attaches to. They tend to carry the pack weight very high, and are cooler than internal frame packs, since there is about an inch separation between the pack and your back. (as compared to internal frame packs which touch your back).
I have both kinds of packs. I use my internal frame pack when I am doing nature programs. I fill it with stuffed animals of foxes, racoons, bears, porcupines, frogs, and skunks. I get the kids attention when I stop and open my back. (I then tell a story about the kind of animal I bring out.) (here is a picture).
The internal frame is fine for my nature programs, but for serious backpacking I always use an external frame. The one reason for this is that with a heavy load, you walk more upright than with an internal frame. This allows me to look straight ahead, and not miss any of the mountain landscape that makes my life so rich.
It is true that you don't have the same stability with an external frame pack as you do an internal frame. To compensate for that, I always use trekking poles. I can't tell you how many times they have saved me from slipping. It is just a lot easier to catch your balance after slipping from four feet with a couple of poles.
Here is another article on external and internal frame packs from a couple of years back.
James E. Ratzloff - I am an avid backpacker who understands that the best of this life is found by following a rocky trail up high into Colorado and Wyoming ...
