A prescription pill that contains a semi-synthetic opioid known as oxycodone. It has (very, very roughly) the same potency as morphine, a more well-known opioid, and is used for the same purpose (relief of moderate to severe pain). Oxycontin in particular is notable for containing no other active drugs besides oxycodone, which makes it very popular with the illegal drug use community. Because oxycontin is a time-release medication, it typically contains more oxycodone than instant release cousins such as Percocet and Percodan. Recreational users are more likely to crush up and parachute, snort or smoke the pill in order to bypass the time-release mechanism. Injecting it is also a known method of administration. Like most opioid-containing preparations, oxycontin is potentially very physically addictive, and that combined with its reportedly euphoric effects have kept demand for it very high since its introduction in the mid 1990's. This in turn has made it one of the most expensive recreational drugs, by weight, with 80 mg pills sometimes costing as much as $100.
Oxycontin is pretty damn awesome, but I'm not an idiot so I'm not gonna do it again for a long, long time. It's also nice, medically speaking, because it alleviates many people's suffering.