From Halopedia, the Halo wiki
Real-time Strategy, or RTS for short, is a genre of computer games characterized by being wargames which take place in real-time, where resource gathering, base building, technology development and direct control over individual units are key components.
Real-time strategy titles do not involve "turns" like turn-based strategy video or board games. Rather, game time progresses in "real time;" it is continuous rather than turn-by-turn, and all players may give orders to their troops at any time. While the word "strategy" originally referred to high-level war planning (armies, campaigns, and entire wars), in real-time strategy games individual units or persons are given orders. Also, integral to the gameplay of real-time strategy games are economic and production aspects (resource gathering, construction, positioning of buildings, expansion, and production of units), and though military confrontation is a significant part of real-time strategy gameplay, it is most often heavily stylized with relatively little emphasis placed on simulating real warfare, in contrast to games of the genre known as real-time tactics.
It is common for games to be miscategorized as belonging to the real-time strategy genre. This is partly because real-time strategy is a vague denomination, giving rise to the assumption that all games involving strategy played in real-time are "real-time strategy" games. It is also partly due to the fact that the genre is so commonly recognized and well-established that there is a tendency to classify many types of games within it. For instance, SimCity, which is a city-building game, Railroad Tycoon, an economic simulation game, and games of the real-time tactics genre of military simulations are often classified as "real-time strategy."
Halo Wars and its sequel, Halo Wars 2, designed by Ensemble Studios, is an RTS game. Halo: Combat Evolved was also originally planned to be an RTS game, but it was converted into a First Person Shooter game.