Swine flu is an Influenza that is "developed" through swine. A pig becomes infected with the influenza and carries it. Hopefully, the swine does not pass the influenza along, but often it will to other swine. Over time, this can mutate and increase the likelihood of passing to a human. At that point, when a human becomes infected, there is a swine flu virus. The fear is that this virus will adapt/mutate and become highly contagious. An example of this situation was the Spanish Flu of 1918. It killed tens of millions of people world wide. Unfortunately, Swine make fantastic breeding places for viruses as they are susceptible to avian (bird flu) flu, human influenza, and swine flu. So, a swine flu might be the next flu to take the world by storm.
Swine flu masks are masks that can be used during a time of emergency or pandemic to help protect you from the swine flu. Flu masks known as respirators that are N95 help filter down to .3 microns. This means they are able to protect against smaller particles in the air and help filter out dangerous materials. Generally, these respirators fit tighter to your face (covering your nose and mouth more closely) preventing extra unfiltered air from getting in. The less unfiltered air, the better.
Swine flu masks should be used with common sense during a pandemic. Proper hygiene such as covering sneezes/coughs, washing hands frequently, avoiding public gatherings, etc will help reduce your chance of getting the flu.
Unfortunately, there is no swine flu mask that will protect agains swine flu completely. You need to use the flu masks along with proper hygiene: wash your hands frequently, sneeze into your arm, don't touch things and then touch your face, etc. You cannot rely on a swine flu mask to protect you 100%. Avoid public areas in case of pandemic. Listen to government warnings and heed them! Stay home when possible and avoid spreading the disease if you are sick.
Again, I stress this point: buy swine flu masks now, before an emergency. During a pandemic, a time of crisis, flu masks will sell out fast. They can be stored easily, aren't very expensive to purchase, and might save your life. Plan ahead, don't count on anybody else during a pandemic. Relying on another during such a time might cost you your life.
During an actual pandemic or emergency, governments will likely be trying to hand out flu masks. Local hospitals or relief agencies might, as well. Use flu masks along with proper hygiene and avoid crowded areas as much as possible.