The fantasy setting of the Warcraft series includes many fictional races and factions. Most of the primary protagonists of the series belong to either the Horde or the Alliance. However, there are a variety of neutral races and factions, who are either friendly or hostile to both the Horde and the Alliance. All player characters belong to either the Horde or the Alliance, with a character's faction decided by its race. One exception is the pandaren, who can choose to become members of either faction.
By the time of Warcraft III the Horde and the Alliance both fight against the Burning Legion and the undead Scourge, who are the primary villains of Warcraft III and World of Warcraft. By the time of World of Warcraft, the Alliance and the Horde are not engaged in all out war any longer. However, they are still hostile towards each other and skirmishes between the two sides occasionally erupt. With Garrosh Hellscream appointed as the new Warchief of the Horde in Cataclysm, war has once more erupted between the two factions. This war plays a large role in the storyline of that expansion, and continues to do so in Mists of Pandaria.
The Alliance has been present in some form in all Warcraft games. In all three real-time strategy games, the Alliance are the protagonists of their campaign, and are one of the two main protagonist factions in World of Warcraft. They are also the primary antagonists of Warcraft and the orc campaigns in Warcraft II. The Alliance began in Warcraft II when the human kingdoms and demihumans strategically united to fend off the conquering Horde. Thus they are enemies to the Horde. The Alliance has evolved over the course of the franchise, losing allies and gaining new members, but the Alliance has endured over the years. They are united to uphold their common noble ideals and are bound together by a sense of brotherhood forged by all the battles they've endured together. The major races of the Alliance are the humans, gnomes, dwarves, night elves, draenei, worgen, and the Tushui pandaren.