How Drugs are Classified & Effects on the Central Nervous System

Drugs can be classified in a variety of ways. The most common classifications are the uses of the drug, effect on the body, source (man made versus extracted from a plant), legal, and risk.

A great way to really view the effects of a drug is to examine how it effects the central nervous system (CNS) and the brain. Here is a brief run down of how the most commonly used substances effect the CNS.

Stimulants
As one would guess by the name, stimulants have a "speeding up" effect on the CNS activity. These drugs increase feelings of alertness and increase energy.
Cocaine
Amphetamines
Pseudophidrine (Sudafed, Codroal Cold and Flu)
Nicotine
Caffeine

Substances such as nicotine, caffeine, and the cold medicines are easily obtained and easily abused. Caffeine is one of the most widely abused substances in the US (however compared to the effects of cocaine and amphetamines is by far less harmful).

Depressants
Depressants have the opposite effect of stimulants. They tend to have a "slowing down" effect on the CNS. The feelings had by the user are: relaxation, sleepiness, or less pain. It is also important to note that the term "depressant" refers to the slowing down effect on the CNS and not necessarily on the mood of the user. In moderate use depressants most likely result in a euphoric feeling.
Alcohol
Major tranquilizers
Benzodiazapines  (Valium, Temazapam, Xanax)
Opioids (Heroin, Morphine)
Other Substances such as: glue, gas, and paint (substances used by inhaling or "huffing").

Hallucinogens
Instead of "speeding up" or "slowing down", these drugs cause sensory distortion in the user (altering the user's senses and perceptions). This is caused by the drug distorting the messages carried within the CNS. Experiences on these drugs are usually referred to as "trips."
LSD (Lysergic Acid Diethylamide) (Acid)
Psilocybin (magic mushrooms; typically grown or found in cow manure)
Mescaline (peyote cactus)

Sources:
Australian Government Dept of Health
http://www.health.gov.au/internet/publications/publishing.nsf/Content/drugtreat-pubs-front6-wk-toc~drugtreat-pubs-front6-wk-secb~drugtreat-pubs-front6-wk-secb-3~drugtreat-pubs-front6-wk-secb-3-1