Silver

Silver, Alchemical: A complex process involving metallurgy and alchemy can bond silver to a weapon made of steel so that it bypasses the protective damage reduction of creatures such as devils and vampires.

Silver weapons have 2 points lower hardness then equivalent steel weapons, and as such aren't quite as durable.

Silver weapons can bypass the damage reductions of Vampires, Lycanthropes and some Devils.

With some devils (Pit Fiends and Horned Devils), the Good damage type may also be needed. See Monster DR and weaknesses for specifics.

Since the Menace of the Underdark expansion, Lycanthropes exist in DDO with the introduction of Werewolf monsters.

Monks with the Shintao Monk III enhancement can bypass Silver damage reduction with unarmed attacks.

The Silver Longbow does not bypass Silver DR on foes as it's simply a material type for the bow, but what counts for bows is the ammunition used. Any bows loaded with silver ammunition do bypass Silver DR. Metalline bows, however, do bypass any metal-based DR, regardless of the ammunition, as metaline is a magical effect that transfers to the ammunition used.

History
In the original release of DDO, silver weapons worked more like their pen and paper counterparts and dealt 1 point less damage per swing. This was changed to simplify the game.