There are basically three types of drugs. 'Uppers' generally make people feel energetic, confident and friendly. 'Downers' generally make people feel relaxed, calm and even sleepy. Hallucinogens generally make people see things that may not be there, or enter a 'dream state' - the hallucinations may be pleasant or they may be extremely frightening, there is no way to tell beforehand what the 'trip' will be like.
Some common hallucinogens are: LSD ('Acid') and mescaline.
Some of the effects of illegal drugs can also be experienced from taking prescription drugs. This is particularly common with strong painkillers. It is just as easy for a person to become addicted to 'legal' prescription drugs as to illegal street drugs.