Backpacks come in a variety of types depending on your use for them.

Short-term hiking packs are generally light packs designed for comfort because of the expected time duration of wearing it on your back.  Backpacks, typically used for school, are usually roomy but without a lot of emphasis on long-term comfort as it is mostly used for minutes at a time before being set down again.  Camping, Survival, and long-term hiking are usually quite large and often have an internal frame to assist with support in carrying it over long distances for prolonged periods of time.
Features of backpacks can vary from being a simplistic device intended to hold objects to more specific functions such as incorporating a hydration sack or a Molle style design that enables you to strap other bags such as day packs to the main pack.

 

Have a look at the wide variety of backpacks available and see which is right for you.
http://www.fcsurplus.ca/shopping/products/47-BAGS-BACKPACKS/