What is bursitis. .? how it comes? is it different from arthritis ? please explain

Bursitis: Throughout the body, there are places where tendons run over or under bones. At these points the tendons can be injured by their contact with the bones. As a result, to protect the tendons, the body has bursas between the bone and tendon. The bursa is a thin filmy tissue. When this bursa gets inflamed, that's bursitis. Arthritis is damage to cartilage in one's joint(s).

Sacs: Bursae are sacs that lubricate any area where muscle passes over bone or under skin and sometimes between muscles. Arthritis is a disease of joints where bones come together.