CBD is one of over 60+ compounds found in cannabis that belong to a class of molecules called cannabinoids. Of these compounds, CBD and THC are usually present in the highest concentrations, and are therefore the most recognized and studied.
CBD and THC levels tend to vary between different strains and varieties of cannabis. CBD does not cause a high, unlike THC. The reason why CBD is non-psychoactive is due to its lack of affinity for CB1 receptors. CB1 receptors are found in high concentrations in the brain, and are the pathways responsible for the psychoactive effects of THC.
Numerous studies suggest CBD acts to counter the intoxicating effects of THC, including memory impairment. However cannabis with both CBD & THC is still psychoactive, just at a different level.
Both CBD and THC have been found to present no risk of lethal overdose. However, to reduce potential side effects, medical users may be better off using cannabis strains with higher levels of CBD.