A drug is a substance which has an effect on the body or mind. This may be a positive effect in the case of medicines or negative as is the case with most illegal drugs.
Since time immemorial mankind has noticed that certain herbs, extracts and other natural substances have positive effects on certain maladies. With the advent of modern science, these have been better identified and tested, and many of the active chemicals involved have been synthesized. A great example of this is aspirin (salicylic acid), which was isolated from the source for which it is named, the willow tree (Salix spp.), and then synthesized in the lab.
There have been many steps along the way which have revolutionized medicine, like the discovery that certain fungi have an antibiotic effect, killing harmful bacteria. The first identified member of this mow large family of drugs was penicillin.
In the last few decades there have been tremendous advances in the invention and manufacture of wholly synthetic substances designed for specific pharmacological effects. Two well known groups of these are the synthetic painkillers and anti-depression drugs (SSRIs). As the details and mechanisms of various ailments become better known, drugs that are more and more specific (having less side effects) can be engineered.
Illegal drugs are drugs that the government has declared to be illegal. This is often based on a perception, true or otherwise, that such substances can potentially cause damage to a person's brain and other organs. Equally there is concern about the fact that most drugs cause addiction. There is much debate over the legalization of drugs, people on both ends of the political spectrum are advocating for some drugs to be legalized while others on both ends are advocating for tougher penalties for possession of drugs. Some new age hippies believe drugs can separate one's body from one's soul, and native Americans believe drugs allow one to talk to trees; current research  in neuroscience has shown that psychedelic drugs can be used to induce religious experiences .
Those in favor of legalizing drugs often argue that the negative social consequences, including gang warfare and the permeation of dangerously impure substances, are more to do with their prohibition than the drugs themselves. Indeed, before the criminalization of heroin in the UK it was no more than a trivial medical problem . Opponents of criminalization also point to the Prohibition experiment in the USA, where the illegality of alcohol caused problems of gang warfare and unsupervised distilleries that made worse the very problem that prohibition was intended to solve.
Illegal drugs in Britain
There are so many drugs on the streets of Britain, "not even the dealers know them all". Popular drugs on the streets of London include "Triple-sods", "Yellow Bentines" and "Clarky Cat".
David Amess MP, the Conservative Member of Parliament for Basildon, warned against the dangers of an Eastern European drug called Cake, and went as far as to ask a question about it in Parliament. The drug affects an area of the brain called "Shatner's Bassoon" and is frequently referred to as "a made-up drug" (a drug not made from plants but made up from chemicals).
All of this is actually from "Brass Eye", a UK television series of satirical spoof documentaries which aired on Channel 4 in 1997. None of it was true. David Amess MP was fooled into filming the video warning against the dangers of the fictional Eastern European drug called Cake. It was all a hoax.
Legal recreational drugs
A small number of drugs are legally tolerated in Western democracies. Examples include alcohol, nicotine and caffeine, whereas other drugs which have less adverse affect on the body, such as ecstacy, are banned due to their social stigma.