Tetrahydrocannabinol or THC is the active ingredient in cannabis. Its Chemical Formula is C21H30O2. It consists of three aromatic rings with a hydrocarbon tail.
The typical effects of THC are placidity and calmness, reduced blood pressure, poor muscle coordination. Its effects are similar to most barbiturates, and under a large dosage has been known to cause hallucinations. THC is not physically addictive, and it is believed to have certain medical benefits, but there are conflicting results as to the validity of this.
Most of the information here is paraphrased from www.lycaeum.org