Sneak attack

Sneak Attack
A Rogue may do additional damage when attacking an opponent if one of the following conditions is met:
 * The target is unaware of the Rogue's presence.
 * The target is attacking another player (possibly under the influence of Intimidate from that player or Diplomacy from the Rogue).
 * The target is under the effects of a successful Bluff.
 * The target is helpless.
 * The target is blind.

Targets immune to critical hits (such as an undead, slime, construct, elemental, or anyone with high Fortification) are also immune to Sneak Attack bonus damage.

Ranged and missile weapons deal Sneak Attack damage only if the target is within 15 meters (45').

Racial enhancements and weapon enhancements that add to your attack or damage rolls in Sneak Attack conditions apply to all characters, whether they're Rogues or not.

Gameplay Implications
Allowing Sneak Attacks to work on targets attacking another player is an important change to DDO from the PnP rules. As long as someone else has "aggro", the Rogue gets a damage bonus of 1d6 per two levels. Even at level 1, that's an average of 3.5 more damage per hit. And you will get that bonus damage for at least two or three hits, typically, since it takes that long for a monster to turn aggro from another player onto you, especially if that player is using the Intimidate skill, which holds aggro for a minimum of 6 seconds on a success.

Although the +2 flanking bonus to melee attack rolls is especially useful to Rogues, who have a lower Base Attack Bonus than the warrior classes, attacking an enemy from behind is not required to score Sneak Attack damage.

Attacking while Sneaking isn't required either! In fact, it's counterproductive to take more than a swing or two in Sneak mode since your attack rate is so much slower than normal. Use it to help make sure someone else gets aggro first, then turn it off when you start swinging. Or simply let other members of your party charge ahead and don't join them until they're engaged.

The "helpless" condition is important to remember. Many spellcasters (including Wizards, Sorcerers, Clerics, and even Bards) can render enemies helpless through direct spell effects like Hold Person, or indirectly through Ability Damaged. This can lead to an unfettered flurry of Sneak Attack damage on that victim and defeat him in short order.

The "blindness" condition is also significant to sneak attack. A blind monster receives sneak attack even if the rogue has the aggro. Blindness can come from spells such as Blind, or Sunburst, in addition to being a special effect from a Greensteel Radiance II weapon, Radiance II Guard (Avaliable on Greensteel Items, or Dragontouched Armor's Sovereign Tier III