THC refers to the main psychoactive (stoning) ingredient of marijuana. THC effects vary greatly depending on the strain and the consumer. THC and other components of cannabis, the terpenes and other cannabinoids (CBD, others) interact in synergy to produce their medicinal and recreational effects. The higher the percentage of THC, the higher the potency (high).
CBD or Cannabidiol is the second most abundant component of most marijuana strains. Some strains are bred to contain more CBD than THC. CBD is highly medicinal, but does not get you high as it has no psychoactive properties. CBD is used to treat many disorders; pain, inflammation, anxiety among many other conditions.