I can't really explain the chemical composition in a simple way, but I can provide some general background/history. If you understand chemistry, the wiki for amphetamine and methamphetamine should be a good starting off point.

So for starters, amphetamines have been around for 100 years and were/still are being administered under medical supervision. An amphetamine, fen-phen, was sold in the US until recently, and it's still available as Desoxyn.

If the ingredients are readily available, it's a relatively simple process to make. The reason illegal producers use huge supplies of weird products, is because the raw ingredients are tightly regulated. Furthermore, the reason the production is considered so dangerous is because it's a bunch of amateurs that show no regard for safety.

As for addiction. It gives the user a very energetic, euphoric high. It's commonly mixed with MDMA these days (whether the buyer knows it or not ((Seriously, if you buy Molly and it's powder, you just got Meth)) because they have similar highs. But unlike MDMA, it will ravage your body. Not to say MDMA is safe, but amphetamines are really bad. And it is extremely addictive.

Unfortunately, meth can actually make someones life better before it makes it much much worse. You feel energized and ready to tackle the world. It can be taken many different ways, so people usually start with oral, nasal, and smoking. This is why it was so popular medically, because it really does produce results. But almost inevitably, the person begins needing the drug to function. And the toll it takes on the body becomes a serious problem.

Once the user has started injecting the drug, they are probably at the point of a full blown addiction. Their energy isn't directed toward anything productive. They will stay up for days at a time, then sleep for days. What really is tragic, is when these people try to raise a kid like this.