"Call of Duty is a first-person shooter video game developed by Infinity Ward and published by Activision in 2003. It is the first game in a series with the same name. The game simulates the infantry and combined arms warfare of World War II. The game is based on the Quake III: Team Arena engine. It was accompanied in September 2004 by an expansion pack, Call of Duty: United Offensive, which was produced by Activision, and developed by Gray Matter Interactive, with contributions from Pi Studios. Call of Duty is similar in theme and gameplay to Medal of Honor, as it is made out of single-player campaigns and missions. However, unlike Medal of Honor, the war is seen not just from the viewpoint of an American soldier but also from the viewpoint of British and Soviet soldiers.
The game is somewhat unusual in that throughout the single-player mode, the player is joined by computer-controlled allies who range in quantity from two infantrymen (in some of the British missions) to an entire regiment of tanks (in the Soviet missions). The computer-controlled allies will support the actual player during the missions. They also further the game's goal of providing an immersive and realistic experience; that is, soldiers in World War II were usually part of a larger group, as opposed to the "lone wolf" seen in video games such as Wolfenstein 3D. However, there are some missions where the player is alone.
Call of Duty also featured "shellshock" (not to be confused with the psychological condition of the same name): when there is an explosion near the player, he momentarily experiences simulated tinnitus, appropriate sound "muffling" effects, blurred vision, and the action moves slower.
The "Hardened" and "Prestige" editions of Call of Duty: Modern Warfare 2, released on November 10, 2009, includes a redemption code to download the game onto Xbox 360 and PlayStation 3 hard drives, retitling it as Call of Duty: Classic."
Source: Wikipedia, "Call_of_Duty_(video_game)," available under the CC-BY-SA License.