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

Selecting the right size backpack is critical. Get one that’s too big and you will have too much storage for your needs. Get one that is too small and you will have to either cut down on the amount of equipment that you bring or strap on so much extra gear that fellow hikers will quickly tire of your clicking and clanking.

Short-term hiking packs are called daypacks. This is because they are smaller in size as backpackers on one day hikes can get away with a limited amount of equipment. Generally these are 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 multi-day 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.