Cannabinoids are naturally occurring compounds found in the Cannabis sativa plant. Of over 480 different compounds present in the plant, only around 66 are termed cannabinoids.
The most well known among these compounds is the delta-9-tetrahydrocannabinol (Δ9-THC), which is the main psychoactive ingredient in cannabis.
Cannabidiol (CBD) is another important component, which makes up about 40% of the plant resin extract.
The cannabinoids are separated into the following subclasses:
Cannabinoids exert their effects by interacting with specific cannabinoid receptors present on the surface of cells.
These receptors are found in different parts of the central nervous system and the two main types of cannabinoid receptors in the body are CB1 and CB2.
In 1992, a naturally occurring substance in the brain that binds to CB1 was discovered, called anandamide. This cannabinoid-like chemical and others that were later discovered are referred to as endocannabinoids.
The effects of cannabinoids depends on the brain area involved. Effects on the limbic system may alter the memory, cognition and psychomotor performance; effects on the mesolimbic pathway may affect the reward and pleasure responses and pain perception may also be altered.
The main way in which the cannabinoids are differentiated is based on their degree of psychoactivity.
For example, CBG, CBC and CBD are not known to be psycholgically active agents whereas THC, CBN and CBDL along with some other cannabinoids are known to have varying degrees of psychoactivity.
The most abundant of the cannabinoids is CBD, which is thought to have anti-anxiety effects, possibly counteracting the psychoactive effects of THC.
When THC is exposed to the air, it becomes oxidized and forms CBN which also interacts with THC to lessen its impact.
This is why cannabis that has been left out unused will has less potent effects when smoked, due to the increased CBN to THC ratio.
source: https://www.news-medical.net/health/What-are-Cannabinoids.aspx