Drugs are substances that influence the state of mind, and that's why they are used. People who use them want to experience the effects of drugs. Some medicines also influence the mental states, like for example, epilepsy medicine, but they are not taken for that reason. Ritalin/Concerta is a medicine, which can be used for ADHD (a disorder of attention and behavior), but is also being used as a recreational drug now. The dose as a medicine for ADHD is, however, much lower than the dose needed to get the effect addicts crave after. We talk about recreational drugs when you use these substances with the intention to get under the influence of them.
Recreational drugs can be divided into groups in several ways; for example, legal (legally admitted) and illegal (prohibited) substances. Alcohol and tobacco are legal in most countries. Hashish, marijuana, cocaine, speed, XTC, ecstasy and heroin are illegal in most countries.
The most frequently used division is the one that looks at the effects of drugs on human consciousness. This gives three sorts of drugs: drugs that sedate or relax, drugs that stimulate or give energy, and those drugs that change your perception (that you see or hear things in a different way than they really are). Most drugs fit in this scheme. Some drugs are harder to categorize.
Drugs that sedate are: alcohol, opiates and tranquilizers (benzodiazepines, sleeping pills), opium, morphine, heroin. Also, hashish is usually used because of its relaxing effect.
Drugs that pep up are: caffeine, nicotine, XTC, cocaine, speed, amphetamine.
Drugs that alter perception are: magic mushrooms, LSD and, in strong or large quantities, hashish and marijuana.
Some drugs are somewhat hard to categorize. Hashish is usually used because of its relaxing effect, but can also alter perception. XTC also has several effects. Rohypnol can both sedate and cause an excited stage.
Tranquializer medicines are also used as recreational drugs. More info.