Cupping is the practice of using suction cups on the skin to treat medical conditions.
It is used primarily to treat muscular problems involving restricted blood movement e.g. sports injuries, soreness of neck or back, tight muscles, etc. The main mechanism of action, much like a massage, is movement of blood through soft tissue areas. However, unlike massage, it can penetrate much deeper and offers significant relief to chronic conditions.
Cupping may also be used to stimulate the nerves along the paraspinal muscles to improve circulation and function to other parts of the body. Use of cups for this purpose requires an understanding of the acupuncture channel system.
The third major use of cupping is to break up phlegm and congestion in the lungs. The back allows for access to the lungs physiologically through the nerves and soft tissue.
It is acceptable for cups to leave "hickies" or suction marks. The more color the more stagnation was brought to the surface (that is a good thing). The marks are safe and usually go away within a few days.
Effective and safe cupping requires training to avoid damaging the skin or causing serious infection. Currently the only medical license that ensures cupping practices related to treatment and medical safety is the Acupuncture License.