Cupping is a treatment based on suction applied to the surface of the body. Where massage manipulates using added pressure, cupping works with negative pressure. The treatment helps to draw out toxins from the tissues.
Well-known and popular in many Asian countries and communities, cupping is not common in the West. Partly that's because it's not easy to find the proper training, and partly because clients are not accustomed to the process and effects.
Cupping can leave marks on the body, which fade after a few days or occasionally longer. They aren't bruises, and they aren't really sore. The marks come from the toxins that have been drawn out of the body tissue. As the health of the area improves, each treatment will leave less marking, eventually none - even with exactly the same duration and intensity of treatment.
To be fair, the appearance after treatment looks rather daunting. However if you judge based on the effectiveness to improve health, you can quickly become a convert.
Read more on cupping, the marks, and benefits - including how it helped athletes at the Olympics.