A life preserver or personal flotation device (PFD) is a device generally used when out on the water in a boat. It might be a belt or a vest- or jacket-like garment, or a life ring meant to be tossed to someone on the water. Its purpose is to keep the wearer afloat should they be so unfortunate as to accidentally end up in the water - by falling overboard, say, or if their boat capsized or sank. The most effective life preservers can keep the wearer face up in the water even if they are unconscious, which is why many of them have a kind of pillow that lays across the wearer's shoulders. Children need different sized PFDs than adults, and ideally every boater should have a properly fitted life preserver for their own exclusive use. Even good swimmers are advised to wear a PFD when boating, for they could drown if their clothes became very heavy when wet, or if they were in the water for such a long time that they became exhausted and could no longer swim.
Life preservers are rated according to their buoyancy; high buoyancy PFDs typically can keep more than 20 lbs (10 kg) afloat; this might not seem like enough for you and me, but as water provides lift, the average adult only needs about half that much buoyancy. The higher the buoyancy, the more bulky and cumbersome the life preserver, so in less risky situations (near shore in calm weather) most people opt for smaller, lighter, less buoyant PFDs. Actually, sadly, most people opt for no PFD at all, for let's face it, life preservers are geeky, uncomfortable, and orange. However, the US Coast Guard estimates that three out of four people who drown while boating could be have been saved if they had worn life preservers. That's why commercial boats and ships, as well as airplanes, are required by law (in many countries, at least) to be equipped with life preservers for every person on board.
Life preserver can also be used metaphorically to refer to something that keeps something going; a bank loan might be referred to as a struggling company's life preserver, or a flu shot a life preserver for a diabetic.
For tips on choosing a life preserver, see
http://www.physsportsmed.com/issues/may_96/pfd.htm