The Diablo 2 Wiki
The Diablo 2 Wiki archive is available for anyone looking for information on Diablo 2. All information here is pre-D2R but contains a lot of useful information that is still relevant. Updated sections for new D2R features can be found on the PureDiablo Diablo 2 section

Deadly Strike

From Diablo 2 Wiki

Deadly Strike is an item bonus that causes the physical damage of your attack to double. The damage bonus is only from the physical damage, and is calculated after all other damage modifiers (strength, dexterity, weapon bonuses, charms, skills, etc).

Deadly Strike works just the same as the Critical Strike bonus, delivered by various skills.

Deadly Strike and Critical Strike never stack to yield 4x bonus, but both properties are checked when present, increasing the odds that something will double the damage of a given attack.