What does the glow plug do?

The glow plug encourages efficient fuel combustion by heating incoming fuel and air, which aids in starting the car engine.

How often should glow plugs be changed?

Glow plugs don't need to be replaced very often, and generally start to show signs of wear and tear at around 100,000 miles. 

How do I know if my glow plugs need replacing?

A fault with the glow plugs can present itself in a number of ways, including hard starting, engine misfiring, rough idling, white or black smoke, and decreased fuel efficiency.

What causes glow plugs to fail?

Aside from general wear and tear, glow plugs can fail due to excessive carbon build-up or heat, damage to the rod, or a broken tip.