||"Meth" is meth-amphetamine, which is a type of amphetamine. The "meth" from Meth-amphetamine comes from the chemical name "methyl". The chemical Methamphetamine is composed of an amphetamine molecule with an additional methyl group attached to its nitrogen (amine group). A methyl is one of the simplest atomic groups which can be added to a molecule: it is a single carbon atom with a set of (usually) 3 hydrogens.|
Take a look at Chem Compare. If you look on the far right end on these images, you can see that there is a "NH-CH3" on the Methamphetamine where there is a "NH2" on the amphetamine. For many of the known psychoactives, adding a methyl group slightly alters the effects, duration, and/or potency. As you start to pay attention to other chemical names, you'll see "meth" show up in many names and this will almost always indicate that there is a methyl group on the molecule somewhere. For Methamphetamine, the methyl allows it a little better fat solubility and thus better penetration into the brain.
Adderall is simply a brand name for a particular mix of different "stereoisomers" (same atoms connecting at the same places, just pointing in different directions in space at one point in the molecule) and salts of amphetamine.
Take a look at
the page describing Adderall for a little more info about this.
Ritalin (methylphenidate) does contain an amphetamine-like backbone, however it is more complex. Take a look at the difference in Chem-Compare. The additional structures on this molecule also alter its interaction with the body and the neurons in our brains. Methylphenidate is reported to have less euphoric effects (some people describe it as 'more dull') than methamphetamine, but every individual is unique in their reaction to psychoactives, so no statement is universally true. While similar in backbone structure, amphetamine, methamphetamine, and ritalin are all quite unique drugs, with somewhat similar, but distinct, effects.
Another thing to note when talking about the differences in the amphetamine-class stimulants is that one of the strange effects of current culture is that particular drugs are demonized in the news, entertainment media, government information, and school curricula. Methamphetamine is particularly demonized, amphetamine somewhat less so, even though amphetamine-related stimulant drugs -- including methylphenidate (Ritalin), amphetamine (Adderall, Dexedrine), & methamphetamine (Desoxyn) -- are commonly prescribed for children from as young as age 3. All three of these can lead to difficult-to-break habits and can become a problem for some people who try them. But the marketing teams of the pharmaceutical companies do what they can to soothe parents' concerns by separating the image of street-speed users from the clean, clinical, healthy use of their products.
All of these substances are swallowed, snorted, smoked, and injected by users (in estimated order of frequency) and all can be dangerous in combinations with MAOIs, at high doses, or at high frequencies of use.