Cupping is the placement of glass, plastic or bamboo cups on specific areas of the body using a vacuum. Traditionally, a flame is used to create a vacuum inside the cup before it's applied to the body. Nowadays, a pump is used instead to draw the air out of the cup. There is no difference except for health and safety reasons a pump is safer.
The cups are stuck to the fleshy body parts and suck the flesh into the cup. It's believed that placing these cups on specific areas of the body helps to move stagnant Qi (energy) and detoxify the body.
Cupping isn't restricted to use in Chinese medicine only. It has a long history of use in the UK and Europe and has been practiced for hundreds of years. It's use in modern biomedicine declined, but still remained popular in East Asia, where it was brought from and reintroduced into westernised countries.
It isn't painful but will leave a small dark, bruise mark on the body for a few days. It's particularly good for relieving muscle tension.