There are two different kinds of backpacks. Of course there is the simple day pack which will carry just enough water and supplies for a day, and then there is the larger pack which would be for through hiking. When purchasing a pack, you may want to get one that has netting, so it doesn't make your back all sweaty. You should also be aware that when you're hiking up or down, it's going to cause some stress on your shoulders and back. You'll be amazed how 10 or 12 or 15 lbs. in a backpack can give you added pressure. Just think how much a camera, binoculars, three bottles of water, and some fruit can weigh after three or four hours on the trail.
