The major differences between the cannabinoids are determined by the extent to which they are psychologically active. Three classes of cannabinoids, the CBG, CBC and CBD are not known to have such an effect. THC, CBN, CBDL and some other cannabinoids on the other hand are known to be psychologically active to varying degrees.
CBD is probably the most abundant cannabinoid, contributing up to 40% of cannabis resin. Interestingly, CBD may actually have anti-anxiety effects and lessen the psychoactive effects of THC.
This means that a plant with a greater percentage of CBD may reduce the intensity of the effects of the THC, which in effect lowers the potency of the plant. Use of a cannabis plant with less CBD has been shown to have an increased psychological impact and result in unwanted effects such as anxiety.
When THC is exposed to air it oxidizes and forms CBN. CBN is only very weakly psychoactive and not unlike CBD interacts with THC to reduce its effects. This is why cannabis that has been left out unused will have increasing amounts of CBN and decreasing amounts of THC and thus lose potency.