THCA is found in abundance in the fresh cannabis plant. THCA has therapeutic effects but does not make you feel "high".
When dried cannabis is heated (by smoking, vaping, or cooking it) decarboxylation occurs. This is a chemical reaction in which carboxylic acids in cannabis lose a carbon atom from a carbon chain. During this process THCA is converted to THC. THC is therapeutic, and is responsible for the psychoactive effect you experience.
Dried cannabis will degrade when exposed to light and oxygen, causing THC to convert to CBN. This and other cannabinoids produced during the aging process, have highly medicinal properties.