There is Black Metal, Death Metal, Melo-Death, Symphonic Metal, Grind-Core, Metal-Core, and countless other sub-genres. Sometimes the difference between these genres is as simple as whether the singer is attempting to sound like Cookie Monster, or to sound like he's dying in a fire. Other times it's as simple as down-tuning a guitar or adding synths. Dimmu sounds (and looks) similar to, say, Immortal, but uses keyboards.
Bottom line, all these sub-genres are mostly an excuse for metal snob elitists to seem smart at parties and on the Internet. And as we all know, the Internets is just a series of tubes, so it doesn't matter.