Glow plugs are a thin pencil like metal rod with a heating element built into it. Lets elaborate a little bit. I am referring to the glow plugs used in diesel engines. They serve a few purposes on diesel engines. Contrary to popular belief, the glow plug is not primarily designed to heat the air inside of the combustion chamber or the walls of the cylinder. It has a very simple purpose. But first we should discuss what a glow plug is.
Once you turn the key on, the glow plug controller supplies power to their heating elements inside of the metal rod. This rod reaches temperatures upwards of 1000 degrees Celsius. If anyone is familiar with working in a steel mill, used a torch, or put metal over a fire then you know as metal gets hot it starts to glow. First as a dark red as it begins to get hot at around 700C, then to cherry red at around 900C, then to bright orange after 1100C. So when the glow plug is hot, it is 'glowing' orange. That's where it got the name from.
The primary purpose of the glow plug is to give the diesel fuel mixture a primary ignition point. Diesel fuel is not nearly the same as gasoline. It has a much higher flash point. To get it to self ignite, it needs to be sprayed as a mist into a cylinder that has a high temperature. The high temperature is formed from extremely high compression of a diesel engine. When an engine is cold, especially in the winter, the temperature of the intake air and surrounding metals absorbs too much heat from the compression stroke to effectively 'light' off the diesel fuel.
Once the glow plugs are hot, you start to crank the engine. The four stroke diesel engine has a cylinder coming up to the top of the compression stroke. The fuel mist is sprayed into the cylinder. If you could actually see the flame pattern, you would see it originate around the hot glow plug. This helps the engine start in cold environments, but it also helps the engine run smoothly during the first 30 seconds or so, which improves emissions.
Block heaters are a similar way to combat very cold winter mornings. They typically hook up to a 120V outlet. They are located inside of the engine contacting the coolant so that they can effectively heat and maintain a warmer block temperature. However, those temperatures typically range from about 35-80 degrees Fahrenheit over ambient temperature depending on the heater so glow plugs are still necessary.