A vaporizer is a device for smoking herbs. It uses a heat source to heat the herb without burning, but rather by vaporizing.
When marijuana is used in a vaporizer, the vaporizer produces a white cloud of THC rich vapor. Since the marijuana is not burned, it is much cooler than a joint and contains less tar.
The vaporizer uses a heating element rather than a flame. If marijuana is heated from 338 F to 372 F, the THC vaporizes. This is not hot enough for it to burn so it does not produce as many of the toxins that burning it does.
Well first of all the vapor is not as harsh on the throat (though the taste can take some getting used to). Burning marijuana requires higher temperatures than vaporization does. The heat of the burning destroys some of the THC (making it less potent).
Not only that but a recent study found that water filtration systems are worse for the lungs than joints. This is because the increase the amount of tars in the smoke per unit of THC (thus one must inhale more tars to get the same amount of THC). This same study found that only vaporizers contain less tar per amount of THC than joints.