This is defilement and black light, as well as certain spells such as evil demonfire, energy drain and vampiric touch.

Their opposites fall under Positive damage. These are wrath, divine power, the wrath commune and I'm unsure about the good aligns demonfire (it's a tricky commune, with some odd quirks). Don't make the mistake of thinking Light falls under positive - Radiance is another kind of damage.

The idea behind positive and negative damage is that they're tied to the Positive and Negative planes. You should be able to find a map of the planes in-game. The easiest to find is the one in Tir-talath's guild. This will give you the RP background on the elements and how the Heavens and Hell play into them as well.

Enjoy.

Yhorian