From the 420 Magazine, which has a wide variety of information, both technical and user-contributed:
Tetrahydrocannabinol – THC
Delta 9-trans-tetrahydrocannabinol – delta-9 THC is the main psychotomimetic (mindbending) ingredient of marijuana. Estimates state that 70 to 100 percent of the marijuana high results from the delta-9 THC present. It occurs in almost all cannabis in concentrations that vary from traces to about 95 percent of all the cannabinoids in the sample.
In very potent strains, carefully prepared marijuana can be 30 percent delta-9 THC by dry weight (seeds and stems removed from flowering buds). Buds are the popular name given to masses of female flowers that form distinct clusters.
Delta 8-trans-tetrahydrocannabinol – delta-8 THC is reported in low concentration, less than one percent of the delta-9 THC present. Its activity is slightly less than that of delta-9 THC. It may be an artefact of the extraction/analysis process. Almost everyone who uses the term THC, refers to delta-9 THC and delta-8 THC combined, as THC.
Cannabidiol – CBD
Cannabidiol – CBD also occurs in almost all strains. Concentration range from none, to about 95 percent of the total cannabinoids present. THC and CBD are the two most abundant naturally occurring cannabinoids. CBD is not psychotomimetic in the pure form, although it does have sedative, analgesic, and antibiotic properties.
In order for CBD to affect the high, THC must be present in quantities ordinarily psychoactive. CBD can contribute to the high by interacting with THC to potentiate (enhance) or antagonize (interfere or lessen) certain qualities of the high.
CBD appears to potentiate the depressant effects of THC and antagonize is excitatory effects. CBD also delays the onset of the high but can make it last considerably longer (as much as twice as long). The kind of grass that takes a while to come on but keeps coming on.
Opinions are conflicting as to whether it increases or decreases the intensity of the high, intensity and high being difficult to define. Terms such as knock-out or sleepy, dreamlike, or melancholic are often used to describe the high from grass with sizeable proportions of CBD and THC.
When only small amounts of THC are present with high proportions of CBD, the high is more of a buzz, and the mind feels dull and the body de-energized.
Cannabinol – CBN
Cannabinol – CBN is not produced by the plant per se. It is the degradation (oxidative) product of THC. Fresh samples of marijuana contain very little CBN but curing, poor storage, or processing such as when making hashish, can cause much of the THC to be oxidized to CBN. Pure forms of CBN have at most 10 percent of the psychoactivity of THC.
Like CBD, it is suspected of potentiating certain aspects of the high, although so far these effects appear to be slight. CBN seems to potentiate THC’s disorienting qualities. One may feel more dizzy or drugged or generally messed up but not necessarily higher.
In fact, with a high proportion of CBN, the high may start well but feels as if it never quite reaches its peak, and when coming down one feels tired or sleepy. High CBN in homegrown grass is not desirable since it represents a loss of 90 percent of the psychoactivity of its precursor THC.