Original: World of Warcraft shares the Warcraft title with the Warcraft series of real-time strategy games, is set in that series' world of Azeroth, and has similar art direction.
Simplified: Warcraft: Orcs and Humans was a real-time strategy game. So were Warcraft 2 and its add-on, Warcraft 2: Tides of Darkness and Warcraft 3 and its add-on, Warcraft 3: The Frozen Throne.