“Unlike traditional smoking devices, which burn marijuana directly, there is no ash or harsh smoke. The devices are fairly simple to operate: Users place a few pinches of marijuana into a small compartment, then press a button to activate a heating element that operates much like a convection oven, stripping the plant’s cannabinoids and releasing a vapor containing just the good stuff. (The portable devices use rechargeable batteries; pricier at-home models run off power cords.) The vapor, while not entirely scentless, is far more discreet than the usual big ol’ dank cloud of smoke. Users report the ability to toke away in public, without fear of alerting every cop within sniffing range.”
http://nypost.com/2013/11/14/pot-vaporizers-let-professionals-get-stoned/