Opiates are so named because they are constituents or derivatives of constituents found in opium, which is processed from the latex sap of the opium poppy, Papaver somniferum. The major biologically active opiates found in opium are morphine, codeine, papaverine and to a lesser extent thebaine, which is not narcotic. Semi-synthetic opioids such as hydromorphone, oxycodone, and hydrocodone are derived from these substances, especially morphine, codeine, and thebaine. Noscapine, along with approximately 25 other alkaloids, are also present in opium, but have little to no effect on the human central nervous system, and are not usually considered to be opiates.

Reference: http://en.wikipedia.org/wiki/Opiate