In simple terms it's the process of gently heating herbs with hot air to release active ingredients, which are boiled off and inhaled in the form of vapour.
Most vaporizers these days use convection to heat the herb, which is downstream in the vapour path, and separate from the heater. This method heats much more evenly and effectively than the conduction type.
The heated air is either drawn through the herb (manual whip) or forced through with a fan. The temperature can be varied precisely with analogue or digital controllers, or by the draw speed and strength in the manual mode. It's all in the personal preference of the user. Some people prefer to select their temperature to control which active ingredients are released, and some prefer a single temperature and vary their draws. Different temperatures release different proportions of active ingredients in the herb. This affects the tastes and the experience in general.