Microsoft Corporation entered the video game industry in 2001 with the release of its first home console, the Xbox. Marketed toward serious gamers, the Xbox became known as the top console for first- and third-person shooters, thanks in no small part to one of its launch titles, Halo: Combat Evolved. In 2004, Microsoft released a highly successful sequel, Halo 2. Again developed by Bungie Studios and published by Microsoft Game Studios, Halo 2 became available for play on both Xbox and the newer Xbox 360, which Microsoft released in 2005. Halo 2 broke sales records, with over $125 million in sales collected within the first 24 hours of the game's release. Microsoft published a PC version for windows in 2007. Like its predecessor, Halo 2 is a first-person shooter game played in a 3D environment through the eyes of protagonist Master Chief Petty Officer John-117, a cybernetically altered supersoldier from the year 2552. Master Chief is joined by his artificial intelligence (AI) companion Cortana, who resides in the neural interface of the Master Chief's body armor. The plot of the Halo series centers on an interstellar war being fought between humanity and an alien alliance known as the Covenant, complicated by a long history between an ancient intellectual race of Forerunners and a parasitic life form known as the Flood. Players enter the game after the events of Combat Evolved, with the Covenant about to invade Earth. While in pursuit of a Covenant Elite trying to escape into slipspace�Halo's version of faster-than-light travel�Master Chief and his crew encounter another Halo ring. Master Chief and the Covenant Elite Arbiter are both captured and end up working together to stop Halo's activation. Most criticism of Halo 2 focused on a cliff hanger ending, which resulted from time constraints during development. Critics and gamers alike praised Halo 2 for its strategic elements, its challenging and varied combat, the superiority of the enemy and ally AI, its special effects, and its immersive and visceral soundtrack. Halo's single-player, multi-player, and cooperative options have also been applauded, with split-screens and/or a system link feature allowing several Xbox consoles to be connected via Wi-Fi for up to 16 players. Halo 2 boasts added weapons, vehicles, the ability to board slow-moving enemy vehicles, hidden content, and several map expansion packs for multiplayer levels. Of Halo 2's added new features, the most popular and important is an online multiplayer campaign through Xbox Live, an online gaming service that provides different social networking and entertainment features, such as headset-based voice chatting. With its new multiplayer capabilities, Halo 2 also introduced a server-finder feature, known as matchmaking. This allows players to select a game type and be matched with similarly skilled players. Skill levels are determined by the collection of achievement points on Xbox 360 games and through Xbox Live, which gives a player their gamerscore of up to 1,000 on any game. Critics agree that this unique matchmaking technology was a turning point for the gaming industry in the early 2000s.