Phenylethylamine is an organic chemical, and the name can also be used to refer to members of the family of chemicals based on the basic structure of phenylethylamine. These chemicals play an important role in biology and pharmacology, mostly due to their resemblance to adrenalin. The basic structure of the chemical is described in its name: phenyl, which is the chemical term for a benzene radical, ethyl, meaning a hydrocarbon side chain, and amine, or an ammonia radical. Phenylethylamine is a relatively simple organic molecule. It should also be noted that its effects are not related to anything intrinsic about its structure: unlike say, cyanide or sulfuric acid, whose activity in the human body has to do with their natural electron structure, phenylethylamines are only active because it resembles adrenalin, which is the substance our body happened to evolve to carry certain messages.
Adrenalin itself is a phenylethylamine, with an addition of three hydroxy groups, two on the phenyl and one on the ethyl. So are two other common neurotransmitters, noradrenalin and dopamine. It is because a phenylethylamine can fit into the receptors for these substances that it is physically and psychologically active. It also can bind to the receptors of another important neurotransmitter, serotonin, (which is somewhat structurally related), although this is less common than interacting The complexity of the phenylethylamines comes from the fact that there are many different type of receptors for these neurotransmitters, controlling different physiological processes, and there are many different types of phenylethylamines. Adrenalin modulates processes from bronchial dilation to sensory input, meaning that a phenylethylamine could do anything from causing wild hallucinations to stopping an asthma attack. Because people have different physiologies, and different subjective experiences, a phenylethylamine may cause slight jitters in one person and a total panic attack in others.
Making different phenylethylamines is a process of substituting different radical groups for hydrogen atoms on the skeleton. Some of the groups that are commonly added are hydroxy, methoxy or methyl. Since there are 11 open spaces on the molecule, and these three groups can be mixed and matched, there are at least hundreds of possible variants, some of which are in common use, and some of which have probably only been glanced at. There is also, as far as I know, no guide for what type of substitutions will provide what sort of effects. I don't know if there is a systemic explanation as to why adding a methyl group to the alpha position and the amine group produces methamphetamine, a very powerful stimulant, whereas adding a hydroxygroup in the same position, as well as to the ring system, produces phenylephrine, a drug that can barely work as a nasal decongestant.
As stated before, because people's mixes of adrenalin receptors are all different, one phenylethylamine can have no affect on one person, while on another person can cause hypertension and psychosis. For this reason, these drugs, like all other drugs, should be used very carefully, even if they are sold as "harmless" over the counter medications.