The original release of Diablo II had five playable character classes.
Those classes were Amazon, Barbarian, Necromancer, Paladin, and Sorceress. Each began with a different skill system and combat identity, from the Amazon’s bows and javelins to the Sorceress’s elemental magic and the Necromancer’s undead summons.
The number increased when the Lord of Destruction expansion added the Assassin and the Druid. That is why references to Diablo II often mention seven classes: seven is the total for the original game plus its expansion, not for the 2000 base release alone.
Class choice affects skills, equipment preferences, and many possible character builds, but every class can be developed in multiple ways. The original five-class lineup helped establish the build variety that became central to the Diablo series.