THC isn't the only sought-after ingredient in cannabis. Cannabanoids are an important factor in providing anti-inflammatory aid to those viewing marijuana as medicinal. The THC (tetrahydrocannabinol) is one of the many cannaboids in marijuana, and the one particularly responsible for getting "high".
If its the high you're after, then you're looking for high THC strains of cannabis. In the left column you will see a list of all strains of marijuana which are high in THC. When looking at a mature ready-for-harvest cannabis plant, the buds will begin producing what appear to be white crystals on their buds. These white crystals are THC.