Blizzard Entertainment's real-time strategy game series StarCraft revolves around interstellar affairs in a distant sector of the galaxy, with three species and multiple factions all vying for supremacy in the sector. The playable species of StarCraft include the Terrans, humans exiled from Earth who excel at adapting to any situation; the Zerg, a race of insectoids obsessed with assimilating other races in pursuit of genetic perfection; and the Protoss, a humanoid species with advanced technology and psionic abilities, attempting to preserve their civilization and strict philosophical way of living from the Zerg. Each of these races has a single campaign in each StarCraft real-time strategy game. In addition to these three, various non-playable races have also been part of the lore of the StarCraft series; the most notable of these is the Xel'Naga, a race which features prominently in the fictional histories of the Protoss and Zerg races.
The original game has sold over 10 million copies internationally,  and remains one of the most popular games in the world. One of the main factors responsible for StarCraft's positive reception is the attention paid to the three unique playable races, for each of which Blizzard developed completely different characteristics, graphics, backstories and styles of gameplay, while keeping them balanced in performance against each other. Previous to this, most real-time strategy games consisted of factions and races with the same basic play styles and units with only superficial differences. The use of unique sides in StarCraft has been credited with popularizing the concept within the real-time strategy genre. Contemporary reviews of the game have mostly praised the attention to the gameplay balance between the species, as well as the fictional stories built up around them.
The Protoss are a race in the StarCraft series. They are composed of two societies, the conservative Khalai Protoss and the exiled dark templar. The Protoss are depicted as a physically strong species with access to advanced psionic abilities. The Protoss are considered the most technologically advanced race of the series and are the focus of two episodes within StarCraft and its three expansions (Stellar Forces, Brood War and Revelations), as well as featuring in campaigns in the authorized add-ons Insurrection and Retribution. Protoss strategy in-game is usually built around the quality of units the player controls rather than the quantity. Originating from Aiur, a planet on the fringe of the galaxy, the Protoss are normally shown in the games and the novels of the series as the nemesis of the Zerg.
The Terrans are a fictional future version of humanity in the StarCraft series. The Terrans are the most splintered of the StarCraft species, consisting of multiple factions that not only fight the other races but each other as well. Considered an adaptive and mobile species, the Terrans are noted in the lore of the series for their ability to quickly access and drain a planet's resources. The Terrans have been dedicated a full campaign in each real-time strategy video game installment to date, and are defined in-game by their specialization of units and tactics of defense and mobility. In the series' storyline, the Terrans are usually displayed as being caught in-between the conflict between the Protoss and Zerg while also dealing with frequent civil wars.
The Zerg Swarm is a race of xenomorphic "hyper-evolutionary" superorganisms; they are the overriding antagonists of the StarCraft series. Unlike the Protoss and the Terrans, the Zerg are not "tool-makers", lacking technological inclination; instead, they "force-evolve" genetic traits by directed mutation in order to match such technology. Operating as a hive mind-linked 'chain of command', the Zerg strive for genetic perfection by assimilating the unique genetic adaptations of "worthy" races into their own, creating numerous different strains of Zerg. Despite being notoriously cunning and ruthlessly efficient, the majority of Zerg species have low intelligence; becoming mindless beasts if not connected to a "hive-cluster" or a "command entity".