When selecting a new backpack you have to consider a number of factors that will affect its fit and performance. Much like your hiking boots having an ill fitting pack, or the wrong pack for the job, will result in a sore back, sore feet, and in extreme cases possible injury. Unlike a boot that doesn't fit well, the wrong pack will beat you death in a matter of minutes on the trail. I've seen strong healthy people brought to their knees in less than two miles, simply because they didn't do some homework before purchasing or selecting their pack. The first thing to figure out is just what you will be using your pack for.
Backpacks fall into three basic categories.
The first of these are daypacks. These are used for short trips, where carrying a lot of gear or a lot of weight isn't a big issue. The next category is assault packs. It can also be used on a weekend trip, or if you practice lightweight backpacking. The final category is called a full pack.