A hot glass cup contains less dense air than its surroundings. If the cup is sealed onto your skin and cools, as the air inside it contracts it forms a powerful suction. This, like other forms of fireplay, looks absolutely wicked--a huge bubble of purple-red flesh is sucked up from your body--but feels surprisingly mild. It's like a really, really big hickey.
And like a hickey, it leaves very dramatic bruises despite inflicting little pain. If for some perverted reason you want to be marked all to shit without suffering, cupping is the way to go.