Cupping hit the front pages of the tabloids when Gwyneth Paltrow sported the marks as fashion accessories in a low-backed top...
http://news.bbc.co.uk/1/hi/health/3879447.stm
It has several uses in Chinese medicine. It can be used to move energy stagnation, often in achey muscles. This can be done using massage oil and sliding the cups. It's a similar feeling to a massage, but sometimes the pulling sensation feels better than pushing.
It is also commonly used to expel pathogens. In chinese medicine these could be in the form of cold (like the common cold), heat, damp, wind to name a few. Although these seem like foreign concepts many people have arthritis which is worse in the damp (this would be a damp invasion) or the heat (heat invasion) for instance. Cupping is used in Easten Europe for this same purpose and I've even seen hieroglyphics of cups on a temple wall in Egypt.
Another (possibly less common) way the can be used is to tonify the energy of a certain organ.
I'm sure there are more uses as there are many branches and theories within Chinese medicine, but I hope this helps with some understanding