Originally Posted by pwerwagn
Its a block heater. It mostly heats up the coolant, but will help keep the oil temp up as well. Its pretty useful if its below ~40* or so.
Even down around 0*, you only need to plug it in ~3 hours or so before you start it. Its a 900watt or so heater (on the 6.0 version at least), so running it all night is useless unless you wanna just use up energy.
What Jeff said, its the engine block heater. Really helps the engine light off like it's summer when plugged in. Its helpful when the temperatures get anywhere close to freezing.
Up in Canada land where I'm from, we leave them plugged in overnight in the winter, as generally, most people aren't awake hours before leaving for work. Won't hurt anything , except use a but more electricity, like Jeff mentioned.
Also, if you ever have glow plug/grid heater problems, this will help with engine starts as well.