A face mask, as opposed to an NBC mask, is a simple paper, cloth, or foam mask that covers only your nose a mouth. It will filter out most particulate matter and smoke while you breathe.
Among face masks there are a wide range of types from those "for occupational use" to those for use to prevent disease transmission such as the H1N1 flu virus. One of the most common types is the N95 mask. The "95" is a rating representing the mask's ability to prevent 95% of particulates from passing through it.
Remember that face masks have limits and are not to take the place of a respirator or self-contained breathing apparatus (SCBA) where required due to the presence of airborne toxic fumes. Face masks are also not designed for all users so they may not fit children. Persons with facial hair such as beards or large sideburns may also not be able to get a proper seal with a face mask. Persons with respiratory ailments may have difficultly breathing through a face mask so they should consult a physician before employing one.