It's a wire that goes to your spark plug. Hence the name "Spark Plug Wire". Interesting eh?
Some have resistors in them, while others do not. They can be between roughly 5k-15k ohm depending on the spark plug wire. If yours doesn't, and you want those electrons to be oppressed, you get a spark plug with a resistor in them. An NGK BR7HS has a resistor in them, while a B7HS does not. I wonder how I can tell them apart?
Some are longer than others.
There's something connected to the other side, but I'll leave that up to someone more qualified than me to sum it up for ya. Best I can do for now? Electrons come from your CDI or points. There's a box in between that and your wire that teaches electrons how to behave, and not to kill everything.