There's a term that I tend to use when describing mechanics in Dungeons and Dragons, and near as I'm aware, it's not used anywhere in the game rules. That word is "instance."
Damage comes in many forms, but a lot of these aren't even officially reconized. You have the "Base damage" that a power deals, ala the [W] damage and dX damage of weapon and implement powers, respectively. You have the ability damage many powers give. You have "Extra damage," which comes from a number of sources.
The significance of a damage instance is that, whenever you gain a bonus to damage, or a bonus to damage rolls, theoretically, it would apply once per damage instance-rather than once per "attack." The significance here is that you'll often have situations like Draconic Arrogance, where pushing or knocking an enemy prone deals damage-not in addition to existing damage (Extra damage), but rather, in and of itself. Any Extra Damage that applies to damage instances (rather than attacks) would apply to Draconic Arrogance's damage.
In short, a Runepriest is a monk's best friend. Pelor's Sun Blessing can be really useful for a Radiant Fist. And don't get me started on the Morninglord paragon path.
But what is most interesting is that sometimes, extra damage occurs when you least expect it, and other times, what you would expect to be extra damage isn't. For instance: Hellish Rebuke appears at first glance to deal damage that is then effectively doubled (sans Warlock's Curse) if you're damaged. But this isn't correct. Hellish Rebuke deals extra damage if you're damaged, meaning it doesn't create a seperate damage instance. But what's utterly freakish is that this non-seperate damage instance occurs at a different time than the power initially deals damage-so a creature effectively retroactively takes more damage, which can create the rather confusing paradox where a creature attacks the warlock, thus meaning that it had effectively suffered more damage than originally anticipated, and drops dead because of it! In practice, the Extra Damage nature of Hellish Rebuke just means that it doesn't benefit from Implement Focus or other bonuses to damage rolls-no matter what, the only damage the target will suffer above and beyond the original damage of the attack will be 1d6+Constitution modifier (or 2d6+Constitution modifier if you're epic).
For the flip side, sometimes you expect damage to be extra damage but it isn't. For instance, a Firestorm Arrow deals 1d6 fire damage per enhancement bonus to the target-which you would expect to be extra damage. But since it also deals fire damage to adjacent enemies, in reality, it's actually a separate damage instance. If you're using a fire damage attack with firestorm arrows, the target's resistance (or vulnerability!) to fire is going to apply not once, but twice! Likewise, a Lightning Weapon's daily item power deals lightning damage to the targets-not extra damage.
Perhaps the most bizarre example I've found is the Tempest Whetstone-dealing extra damage to a target that wasn't damaged in the first place. Theoretically, this would mean that vulnerability and resistance don't trigger-or maybe that the original target's resistance and vulnerability trigger for them! But that's absurd; a more reasonable assumption would be "This doesn't trigger bonuses to damage" (such as Headsman's Chop).