Unlike their counterparts the spark plugs, glow plugs don't use a spark to ignite the fuel, they use heat. A glow plug is the closest thing to a perpetual motion machine. Initially you ally a current to the plug, with a glow starter, also know as a glow warmer. this makes the filament glow red hot. You then turn over your engine, as the fuel comes in contact with the hot filament and it is compressed by the piston it ignites, forcing the piston down, the heat of the combustion will keep the glow plug filament glowing.

Because of the way in which this works timing is handled automatically by the engine, as you rev higher the filament gets hotter and the fuel/air mixture ignites earlier, as the engine idles the opposite applies.

In my experience a glow plug should last between 10 and 20 runs, although I have had some that seem to run forever there is a definite drop in performance near to the end of it's life.

The glow plug can tell you a lot about how close to perfect your air/fuel mixture is, see the glow plug reading guide for more information on how to do this.