Start with questions for yourself.
Do you need a backpack?
There are other options, not the least of them is the "rickshaw" or "game cart". You can carry more weight with a rickshaw cart, but the wheels can limit you in the locations you can go, a backpack will go where you go, but you will not be able to carry as much weight.
How much "stuff" will I need to pack?
How many pockets and how much do you need to carry? If alone, you will carry more, but take a hint and drop as much as you can, carry only what you need - study the ultra-light backpackers and the "live on the land" type of trekkers for tips and tricks.
Think about what you need, and consider outside strapping and internal size that may be required.
There is a general "rule of thumb" you will see with pack manufacturers but I find that if you do it correctly you can go the next size under.
A single "day hike" or "overnighter" to about one week (adding food from plants or fishing/hunting) or in places where you need tools or equipment would require packs in the range of 1,800-2,500 cubic inch (30-40 liters) internal space. Adding outside strapping for odd tools this need could be less than half for short trips...
A pack in hike where you will be spending more that two days you may want to consider a pack in the size of 3,000-5,000 cubic inch (50-80 liters), this is a relatively large pack, personally I have never had a pack larger than 4,800 cubic inches and you can keep up with three weeks of food (if you are adding to that with local plants and or fishing/hunting).
Packs that are more than 5,000 cubic inch (80 liters and more) capacity would be almost "expedition" level packs, or packs where a large amount of tools, shelter, or clothing would be required.
What type of backpack?
Any pack (regardless of type) that is adjusted incorrectly will cause you pain and be difficult to take for long periods of traveling.
All packs are designed to place a majority of the weight on your hip belt! Both internal and external frame packs should have tight fitting, well padded, well designed hip belts! DO NOT depend on the shoulder straps to carry the weight for long periods of time.
There are two basic types of advanced backpacks the internal frame type and the external frame type.
Internal frame backpacks.
Internal frame backpacks are currently the most popular, take care to fit your pack correctly as an internal frame MUST BE adjusted correctly for you, and often you will need to try several packs out to find one that "fits". It is harder to find a well fitting internal frame pack than you would think - go to a store to check them out, be aware that many who will "help you" are one or two season packers, they may not take into account your needs for all seasons. "Four season" equipment is designed for Winter PLUS the other seasons, all other "season" levels are for less demanding camping seasons. Remember camping is considered a recreational activity in the US, take this into consideration when you get advice from well traveled backpackers.
The internal frame packs are typically lower slung, narrow, and will balance well for walking, even when climbing hills and rocks or steep trails. You will be likely to travel along narrow trails quicker and the pack will allow you to even stoop, crawl, or drag yourself in a low crawl without removing the pack (if the space is large enough for you and the pack, it is closer to your body but it does not disappear).
Drawbacks to internal frame packs are related to its lack of a frame, it sits closer to your body and will cause you to perspire, this can be a killer in the winter if you wet your clothing, I made a trip many years ago in winter and got wet, that first night was the most miserable I have ever experienced, I could not get warm and I did not have "night dress" or a bag liner so I could strip down and get the wet clothes off... That was also the last internal frame pack I ever used in cold weather.
Internal frame packs will also not carry as much weight as an external frame pack will, you also must place the heavy items to the bottom of the pack to keep the balance internal frame packs are capable of providing.
Because they are popular, there are many internal frame packs to choose from, the models used by the military are heaver, poorly designed, and unless you happen to be "standard size" poor fitting, for the money there are many choices available that would be a better fit and design (they just come in crap colors - this is an easy fix with a waterproof pack cover).
External frame backpacks.
External frame backpacks over the years have lost their popularity, with the exception of winter trekkers and hunters.
It is easier to find a well fitting external frame pack often getting a great match without having to go to a store to check twenty pack brands and sizes. External frame backpacks just as internal frame are not specifically rated by season, but there are reasons I would place the external frame pack style as a "Four season" pack. Winter has its own sets of demands and ventilation away from winter clothing is the number one reason for an external frame pack, with the ability to carry much more weight in a more "hiker mule" fashion.
The external frame packs typically ride much higher on the back, are wider. External frame packs will balance high and are great for heavy weights on more even ground, they are not particularly well suited for climbing hills and rocks or steep trails. You will be forced to consider travel along narrow trails with detail and forethought, because a high riding heavy pack will require you to remove the pack for small spaces and may even have to drag a pack behind you if the space is low enough.
Drawbacks and benefits to external frame packs center on the rigid frame. The rigid frame will help control perspiration by keeping the pack off your clothing and body except for the main contact points. In winter a rigid frame pack is almost a requirement. The pack is removable from the frame and the frame can be used to pack our large items - one reason the external frame packs are so popular with hunters.
External frame packs will easily carry double the weight an internal frame pack is capable of carrying and will carry that weight more comfortably for the user. Be careful not to place too many heavy items at the top of the pack, an make your self any more top heavy and off balance than is necessary..
Because there are fewer models of external frame packs available, there is a huge price varriance, strangely some of the best designs for external frame packs are the LEAST EXPENSIVE! The models used by the military are heavy, pathetically ill designed, and poor fitting. Just as with internal frame packs there are many superior packs for the money than are available as military surplus.
I suggest some of the least expensive packs as starters, as strange as it sounds I have come full circle back to the inexpensive external frame packs as my main use pack now.
I use the Outdoor Products "Saturn" external frame style pack, as the frame is flexible and I broke my last Cabela "Extreme Alaskan Outfitter" when I fell back on it against a rock wall - that is the last expensive pack I get, next trip I used my son’s Outdoor Products "Saturn" and decided I would get one for myself - a bit heavy (5.5 lbs empty) but flexible and I enjoy it far more than any other external frame pack I have ever used.
There is no need to pitch out five to seven hundred bucks for a pack!
Try this link for Outdoor products inexpensive but good and useable packs, from one of my favorite camping suppliers on the web - Campmor.
Campmor Outdoor Supply