From Diablo Wiki
Attack is one of the basic abilities possessed by all characters in Diablo 2. It's the default left click skill, though it can be set to the right click as well.
- Melee: When a character is equipped with a melee weapon, a melee attack will be launched at the targeted enemy.
- Open Handed: A character without a weapon will punch the target.
- Throwing Weapons: Characters equipped with ranged weapons that can be used for melee combat, such as throwing weapons, will use them for a melee attack. (Use throw to throw the weapon.)
- Ranged Weapons: Ranged weapons other than throwing weapons, such as bows, crossbows, and throwing potions, will all be thrown/fired from a distance, when used with Attack.
- Skills: Skills set to the left click will work like Attack. Melee skills will be used at melee range, while ranged or missile attacks will be fired from a distance.
Most players substitute more powerful special skills for the basic attack, at higher levels. Spell casters usually select a ranged attack spell, such as Glacial Spike, for the left click, to avoid accidentally running up and launching into melee combat they are unsuited to.