In very general terms:
CBD is a semi-legal anti-inflammatory, anticonvulsant, possibly tumor suppressing, pain reducing compound found in cannabis.
Delta-9 THC is a psychoactive compound found in cannabis, responsible for the high.
Both are found in the plant and breeds have been developed that contain high amounts of either or both kinds. CBD is semi legal, in that you can buy it, possess it, but the process of producing it involves growing cannabis which is still illegal for those of us living in the dark ages. Also, most CBD extract or tincture has some amount of THC present. Not enough to get a buzz, but enough to fail a drug test or catch a felony for possession.