An<unk>  is a chemical substance that has a morphine-like action in the body. The main use is for pain relief. These agents work by binding to<unk>  receptors, which are found principally in the central nervous system and the gastrointestinal tract. The receptors in these two organ systems mediate both the beneficial effects, and the undesirable side effects. There are a number of broad classes of narcotics:
natural opiates, alkaloids contained in the resin of the opium poppy including morphine, codeine, thebaine, and oripavine;
semi-synthetic opiates, created from the natural<unk> s, such as hydromorphone, hydrocodone, oxycodone, and heroin;
fully synthetic<unk> s, such as fentanyl, pethidine, methadone, and propoxyphene;
endogenous<unk>  peptides, produced naturally in the body, such as endorphins, enkephalins, dynorphins, and endomorphins.
Although the term opiate is often used as a synonym for<unk> , it is more properly limited to the natural opium alkaloids and the semi-synthetics derived from them.
Amongst analgesics are a small number of agents which act on the central nervous system but not on the<unk>  receptor system and therefore have none of the other (narcotic) qualities of<unk> s although they may produce euphoria by relieving pain -- a euphoria that, because of the way it is produced, does not form the basis of morbid seek orientation, habituation, physical dependence, or addiction.  Foremost amongst these are nefopam, orphenadrine, and perhaps phenyltoloxamine and/or some other antihistamines.  The remainder of analgesics work peripherally.  Research is starting to show that morphine and related drugs may indeed have peripheral effects as well, such as morphine gel working on burns, but peripherally-acting analgesics include aspirin, paracetamol, ibuprofen and the like.
Many of the alkaloids and other derivatives of the opium poppy are not<unk> s or narcotics; the best example is the smooth-muscle relaxant papaverine. Noscapine is a marginal case as it does have CNS effects but not necessarily similar to morphine, and it is probably in a category all its own.
Finally, some<unk> s such as diphenoxylate do not possess analgesic action, narcotic or otherwise.
Opioid
Heroin
Opiate
Cannabinoids
Benzidine
Nicotine
Nicotine 
Phencyclidine