The reason there are many different types of back injuries lies in the complex way your back is made. Unlike an arm or a leg that have bone, muscles, tendons, and ligaments, your spine has many small pieces of bone (Vertebrae), muscles, joints, ligaments, discs, and roots. And, the many discs make for many openings for bending and twisting, giving more leeway for injury.
If the larger roots of the spine are irritated or injured, you could feel pain in your arms or legs, instead of the back. If the smaller nerves along the spine are irritated, this can hurt the back itself. Then, there are the muscles and ligaments that can be pulled, stretched, or even torn; the joints could become swollen, and discs may move out of place.