The volatile oil (mix of THC and other cannabinoids) found in resin glands transforms into vapours at a temperature of approximately 190 degrees Celsius (375 degrees Fahrenheit). Without burning the plant matter itself, this THC vapour is liberated from the cannabis foliage. Like smoke, the vapour can be inhaled and the active ingredients are delivered in the same way by the lungs into the bloodstream.
The main difference between vapour and smoke is that there are little to no carbon particles in vapour and thus, small concentrations of tar and smoke-related carcinogens cannot enter your lungs. Also it is absorbed much more effectively when THC is heated rather than burned, so less cannabis is required to achieve the same result. It is an incredibly cleaner way to intake marijuana.