Backpack
Nowadays the most popular backpacks (also know as rucksacks) are so called anatomical models. The more traditional model is backpack with an external frame, which has lost its popularity lately. Anatomical backpacks are getting better and better so it's a safe choice for everyone.
For a little shorter trek a backpack with a capacity of 50 liters (3 050 cu. in.) may be big enough. For a one week or longer trips the capacity should be at least of 70 liters (4 270 cu. in.). Naturally the need for capacity varies between people depending on personal needs and how much gear you're going to take with you. The weight of the backpacks mentioned earlier starts from about two kilograms (4.4lbs.). There are available some lightweight models which can carry that 70 liters but the weight is still managed to kept well under 2kg. If you're a backpacker aiming at lightweight gears, backpack is one of the first gears to save some valuable weight.
Backpack
When you're buying a backpack, try it in the store with some weight so you get some real feel of the pack. Test the hip belt carefully; it should feel comfortable right away. Some models have even a custom-moldable hip belt which naturally makes the carrying of the pack even more pleasing experience. Check the overall build of the pack, it should be very well build and the fabrics should be highly durable and water repellent.
Many of the packs have integrated rain cover but if not you should buy one. In addition some models have a place for integrated hydration system. Remember that the hydration system itself is not included with the pack.
Packing the backpack
The heavier gear should be packed in the large middle section of the pack and closest to your back. This way the weight is distributed evenly and the backpack won't stress your back too much. Sleeping bag is usually packed in the lower section and the sleeping mat outside the pack, to the top or the bottom of the pack.
The gear which you will need during the day, like food and cooking gear should naturally be packed to be easily and quickly available. When a good quality backpack is packed carefully and adjusted properly it's a pleasure to carry even for a longer trek.
