Most often: muscles. The common, every day type of back pain is usually from the strain on the muscles and ligaments of the lumbosacral spine. Underlying arthritic changes can be the cause of chronic back complaints. If the symptoms travel down the leg(s), the disc structures of the spine can be involved. If ice/heat, short rest, and otcs cannot treat the pain, seek further evaluation by your doc.
Looooooong list. Low back pain is among the most common complaints for which people seek medical care. The list of causes is incredibly long but can be broken down into musculoskelatol causes and organic (from the organs) causes. These include sprains/strains, joint misalignments, kidney problems, cancers of various kinds, etc. Treatment depends on the diagnosis/cause.
Multiple. There are multiple potential causes for low back pain, among the more common include degenerative disc disease, spinal stenosis, facet arthropathy, si joint problems, disc herniations, instability to name a few. Don't hesitate to be seen for an accurate diagnosis and to discuss treatment options.