I think the descriptions are somewhat vague, from what I gather it's probably one of the following:

 

1) Increases all damage of the picked type

 

that's what the description on the character creation seems to imply:

"Druids select a damage type that is added to any attack they use that inflicts damage of one of those types to a target"

 

2) adds damage of the selected type to the next auto attack (after a spell cast?)

 

when creating a druid and hovering the icons the tooltip reads:

"The druid's knowledge of the [damage type] allow him or her to inflict additional [damage type] to the next attack"

 

3) adds damage of the selected type to all auto attacks

 

On the in game character sheet under active effects the effect is described as:

"5% Damage as [damage type]"

 

 

I can't find very compelling evidence for either by looking at the combat logs.

I think it's #3

Edited September 4, 201411 yr by Sensuki

Yes, it's supposed to be #3.

It just didn't seem to do anything before 278 at least, and numbers were all crazy (500% wildstrike, 10000% paladin's flaming strike)