Repost from WoW Vault (Zoe) 

Basic Concept:

The basic concept of tanking consists of three parts, maintaining the aggro, mitigating the damage and staying alive. There are three classes considered to be tanking class, the Warrior, the Paladin and the Druid. Though any class has some form to maintain aggro, mitigate damage and staying alive, only Warriors, Paladins and Druids have skills and talents specific for this purpose, as well as items made for them. This guide only talks about Warriors, Paladins and Druids.