A vaporizer is a device used to extract the active ingredients from plant material without burning it in flames.
A vaporizer heats up the herb using a heating element. Herbs can reach temperatures of up to 1000°+ when burned, so you end up inhaling stuff that you don't need to be inhaling such as plant leaves and carcinogenic toxins. Vaporizing eliminates that only only lets you inhale the active elements that you want. Vaporizers turn the active ingredients into thin vapor at temperature of about 190°.