Unique to mountain environments, avalanches are perhaps the greatest danger in the vertical world (probably my worst fear in the mountains anyway).
The most destructive type of avalanche, due to the enormous amount of snow involved, is called a slab avalanche. This happens when there is a sudden dislodging of overlying snow pack from an underlying layer of snow or ice with structurally different characteristics.
If the snow is new and fluffy and the bond between the individual snow crystals is sufficiently weak, this may cause a slide as well. This is often called a loose avalanche, and is commonly triggered by human activities such as skiing, snowboarding, etc.
Although there are some basic things I can tell you for survival if you're caught in an avalanche — like using swimming motions to try to stay near the surface, or creating an air space with your hands and arms in front of your face as the avalanche comes to a stop, or traveling along a ridge versus a slope to perhaps avoid being caught in an avy in the first place (however, on ridges there are other dangers such as cornices ) — I don't want to give anyone a false sense of security.
Because of the extreme danger of avalanches, I highly recommend taking an avalanche safety course before traveling in avalanche prone areas. You just simply cannot learn the critical stuff by reading an article; you have to get out and get your hands dirty.
A great place to learn is in the hands-on courses given by one of the many mountain-climbing schools.