The AvaLung is a snorkel-like device that you put in your mouth. You then breath in oxygen-rich air from the front of your body and exhale the CO2 saturated air near your back. (The excessive CO2, not the lack of O2, is what asphyxiates you.) The challenge might be getting and keeping the mouthpiece in your mouth, with several documented cases where this was a problem, but there are also several documented cases of survival due to the AvaLung.
The AvaLung can be purchased as a standalone unit, but it's more commonly incorporated into a backcountry-specific backpack.
In 2010 Black Diamond announced a recall of some AvaLung packs because the intake tubing could crack under extremely cold temperatures.