A traditional block heater is the heating coil that fits in one of the freeze plug holes. You plug it in and it keeps the coolent in the block at or near operating temperature.
I think the one you're thinking of is the recirculating heater. It's spliced into one of the heater hoses. These usually include a small pump in them to heat up the coolent and recurculate it through the entire cooling system.
The heating pad on the oil pan is just an oil heater. Using a good low grade or synthetic oil in the winter won't need the oil heater.
Old air cooled VW's had an oil heater. It was simply a dipstick with a heating coil on the end.
As long as you're antifreeze is at a low temp (usually -45) you're block won't crank. Trying to start a car with a cold engine and cold coolent is hard on the starter as well as the engine. Using a block heater just reduces the strain on the rest of the components when you're trying to start the engine that's well below freezing.
I usually don't plug my truck in until it gets below -20c.
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87 IROC-Z SuperPro ET Bracket Race Car
461 naturally aspirated Big Block (times are for the current engine)
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I was going to dare to plug it in either... but when we pulled the engine, I saw that it lead to the oil pan or so... like a heating pad for that I guess.
