Think of the glow plugs as performing the task of a block heater - but instead of heating the water jacket around the cylinders they warm up the air in the combustion chamber.
As mentioned above by layback40 heat is produced during the compression stroke. (Quickly) Compress air and it gets hot. (Quickly) expand air and it gets cold => don't hold onto a nozzle of a CO2 fire extinguisher and release the gas!
Even though quickly compressed air gets hot; unfortunately on a cold engine this heat buggers off quite quickly into the surrounding parts of the engine. The glow plugs give a bit of a boost. They are only in use before the engine starts - they are not used like a spark plug (or the model aircraft example you gave) to ignite the fuel once the engine is running.
Here's a nice link telling more
Indirect injection - Wikipedia, the free encyclopedia