Psychoactive substances are chemicals that react in the brain to alter its normal chemistry. Most (if not all) recreational drugs, such as LSD, "magic mushrooms", and cannabis (to a certain degree) are psychoactive substances. Anti-depresants (such as Prozac, which is an
SSRI) and other perscription drugs are psychoactive.
Outside links:
For recreactional drugs: http://www.erowid.org