Plants in the genus Cannabis contain unique compounds called cannabinoids. There are at least 113 different cannabinoids produced by cannabis plants. The most notable of these cannabinoids is delta 9- tetrahydrocannabinol, also known as THC. THC is the primary psychoactive compound found in marijuana. While marijuana plants contain high levels of THC (typically between 5-25%), the varieties used for hemp contain very little. Hemp has been selectively bred to contain no more than 0.3% THC on a dry-weight basis.

 

Source: Massachusetts State Gov. website