Feat:Sneak Attack

From DDO Compendium

Feat: Sneak Attack Allows you to deal additional damage when a monster is unaware of your presence or targeting another character.

 

Sneak Attack damage is added to normal physical damage (ie. it is subject to DR). Even though it is displayed separately & red in "floaty numbers", it is added to your physical damage in the combat log.

Sneak attack only applies against targets that are susceptible to critical hits, and can be blocked by fortification. It applies only to ranged weapons when you are very close to the target.

A rogue gets +1d6 sneak attack damage at every odd level (+1d6 at lvl1, +2d6 at lvl3, and so on). Sneak Attack damage can be further improved by rogue enhancements, or items "of Backstabbing". A rogue with the "Wrack Construct" enhancement has the ability to do 1d6 extra damage and to make constructs make a reflex save vs DC 10+1/2 Rogue Level+Int bonus or be 10% more susceptible to Critical hits for a short time.

Sneak Attack (thus rogue level 1) is a prerequisite for the Hamstring feat.

Sneak Attack is applied whenever you meet any of the following conditions:
1) you do not have aggro (ie the monster is not attacking you).
2) the target is blind, held, stunned, or many other effects.
3) the target is at 0 str or 0 dex.
4) target is bluffed (target will no longer be bluffed after 1 sneak attack)
5) many other situations i probably missed.