For those of you scratching your heads right now, "texture" means image (think textures on 3D game objects), and "alpha" refers to the transparency of the image. An image with an alpha of 0 is completely transparent, while one with an alpha of 1 is completely opaque.
Trigger conditions for displaying an aura include: - Presence/absence of a buff on yourself, enemies, or party member s - Presence/absence of a debuff - Power levels (energy, mana, rage) - A skill being usable (i.e. off cooldown, and active for active skills like Overpower) The last one I find especially useful for my Destruction warlock; I have a green dragon show up to the left of my character when my Chaos Bolt is ready to fire, and an orange flame to the right when I can Conflagrate. Throw in a timer bar for Immolate, courtesy of Elkano's BuffBars, and I've got all the information I need right in the middle of my screen.
Of course, since I like information overload, I also have DoTimer cooldown bars for CB and Conflag, but they're not strictly necessary. That's the one feature I miss from PAC, actually - I'd like to be able to show an aura if a skill was going to be usable in 0.5 seconds, for instance, or at least start it fading in at that time. There is apparently some way to make PAC show you countdown timers, but I haven't figured it out yet.
Anyway, if you think it might be helpful to have customizable indicators that light up under certain conditions, PAC is for you. I was intimidated by its complexity when I first looked at it (and I'm somebody who swears by Grid and PitBull, and misses FlexBar like crazy), but it's really not that bad. It's set up to make it easy to do common tasks (like everything should be). Good job to the creator/maintainers for making it about as usable as could be for a low-level display addon like this.
Download Power Auras Classic at WoW Interface