There are two main active ingredients in marijuana plants. Actually, there are a total of over 100 active ingredients, called cannabinoids, but there are two that are the most important. Everyone has heard of Tetrahydrocannabinol - THC, which is the drug that gets people high. Cannabidiol, or CBD, is the other main ingredient.
CBD has almost no psycho-active properties, so it doesn't get people high, but it seems to have some quite powerful medical effects. There are no comprehensive studies yet done on CBD, but a number of preliminary studies have shown some quite exciting results. CBD may be very helpful for seizure disorders, pain and anxiety problems. A CBD product was recently approved here in the US for Dravet Syndrome, which is a rare condition that causes intractable seizures starting in infancy. Hopefully, continued research of this compound will uncover more about its usefulness and how it can help people.