"One-handed swords are sharp blades held by their hilts. They’re truly versatile weapons, used effectively for slashing, thrusting or hacking at exposed, sensitive flesh."— Game Guide(src)
A Sword is a type of sharp-edged weapon seen throughout the Diablo series of games. Swords come in many forms, shapes, and sizes. In Diablo I and Diablo II, both one-handed and two-handed swords are classified as single weapon type, but in Diablo III a separate type of Two-Handed Swords exists.
In Diablo I, Swords are mostly used by Warrior, Barbarian and Bard. They deal bonus damage to Animal enemies, but are less effective against the Undead. Daggers are considered a subtype of Swords in this game.
The two-handed swords prevent heroes from carrying a Shield, but are animated as one-handed still. Barbarian may wield those in one hand however.
In Diablo II, many Swords have two profiles: they can be wielded in one hand by the Barbarian. Other swords are always one-handed. Swords from Diablo II do not have any special damaging effects against either type of enemies however, and are separate from Daggers.
In Diablo III, Swords are faster than Maces and Axes, but the average damage per second of all three types is roughly the same. All three weapon types are free to be transmogrified into each other.
Mighty Weapons also exist in sword variants.