Summarize the following:
Not all hiking trips will be the same and there are varying styles of backpacks that can meet the demands of these differences.  Depending on the length of your hike and the gear you need to carry you will need to purchase a backpack with the correct frame style.  Backpacks with external frames are mainly used for heavy and asymmetrical loads. An internal frame backpack might be right for you if you need to maintain stability and don't need to carry heavy items. Frame-less backpacks are for those hikers who don't need to carry much gear with them and like to move quickly on their hike. Hiking backpacks will need to be fitted exactly to the size of your torso.  Any backpack that isn't the right size won't fit correctly and will be uncomfortable or may cause injury.  Measure your torso's length and hip width before you buy your backpack to ensure a great fit.  Measure your torso's length by starting from the top bump at the base of your neck down to your hip level. Measure your hips to make sure that the hip belt will fit.  Your hip measurement will be taken just above where your pants normally rest. Before you load up your backpack and head out hiking you will need to properly fit your backpack.  Try wearing your backpack and then making adjustments using the straps on your backpack where needed.  There will generally be four main areas to adjust including the hip-belt, shoulder straps, load lifters, and sternum straps.  The hip-belt should fit snugly around your hips and will support much of the weight. Shoulder straps should pull the bag tight against your back but not actually carry much of the weight. Some backpacks will have load-lifting straps.  These should relieve the weight from your shoulders and help keep it properly distributed. The sternum strap will pull the weight up and away from your shoulders and should fight tightly and comfortably across your chest. When it comes to hiking backpacks, it is ideal to visit a store where you can try on the backpacks in person.  Although you might know your measurements, each backpack might fight differently in actuality.  Try on plenty of backpacks before purchasing one to make sure you find the right fit. If you are unable to visit a store you may still be able to try a backpack and return it free of charge if the manufacturer allows for it.

Summary:
Think about what type of hiking backpack you need. Measure your torso. Fit your backpack. Try on the backpack.