The "Command & Conquer 3" CD key is used to install the "Command & Conquer 3: Tiberium Wars" computer game. Product keys validate that the game's owner purchased the game legally. Without a product key, the player cannot install the game.
Product keys are used to protect computer software from piracy and illegal trading. The programs that use product keys range from computer games to security systems and even operating systems. The keys are generated uniquely with mathematical algorithms, making it necessary to purchase the software legally prior to installing it rather than finding a key that is already used.