In order for vaporization to occur, your herbal blend must reach a certain temperature. The temperature must be low enough that your plant material does not burn, but high enough for the herbal essences to vaporize. The temperature of vaporization differs between the various plant materials and might require experimentation until you get it just right. (For a table of vaporization temperatures for various plants, check out this article: Table of Vaporization Temperatures)
There are two main methods of heat transfer currently used by vaporizers, Conduction and Convection.
Because this isn't a thermodynamics class, we'll keep it simple with a summary of the two differences.
Conduction: Conduction heat transfer is heating of matter through direct contact. Basically, this means that a heating element transfers heat to whatever it is touching. In the case of vaporizers, the heating element bowl (usually brass) heats up and vaporizes the herbal blend to release only the herbal essences. Unlike burning, vaporizing produces a clean potent vapor free of tar and other carcinogens.
Convection: Convection heating is when an object is heated via 'fluid', which usually means 'air' in layman's terms. In convection heating, air passes though a heating element (usually ceramic) and is heated to the desired temperature. Then, the heated air is passed through your herbal blend, vaporizing the essences and delivers a clean vapor that is free of tar and carcinogens just like a conduction vaporizer. (Examples of convection vaporizers: Silver Surfer Vaporizer, Zephyr Ion Vaporizer, Da Buddha Herbal Vaporizer)
(Example of both convection and conduction vaporization: Vapir NO2 Portable Vaporizer)
Primary benefits of conduction heating
Primary benefits of convection heating