You attack if you apply damage or a debuff with any of your active skills or Weapon Pool Skills (WPS).
The basic attack is also an attack for the purpose of triggering procs, but you cannot bind a celetial skill to it.
Auras that attack enemies can only be used with celetial skills which are triggered on (you being) hit or on block.
Proc’ed effects do not trigger additional procs.
The forum search is pretty good, GD’s Game Guide and especially the page about Combat is a useful starting point - there you also find an explanation for % Current Life Damage (aka x% Reduction to Enemy’s Health) - and if you need to dig deeper you can find Mechanics Guides and other tutorials. A regular web search would only be my last resort.