Spies are secret agents that work for some kind of agency. What they must do is get behind enemy lines and gather enemy intelligence. They can do this by breaking into an enemy base or disguise themselves. They often work alone and use stealth. They also use a wide array of gadgets from hidden microphones to shoes that can also be use as phones. If a spy gets caught he's basically dead since he works without backup. He's a one man army.
Video games often have you playing as a spy. These games usually involve the player sneaking his way around, grabbing enemy intel and using a silenced pistol. These games are usually associated to the stealth genre. The spy may also be a multiplayer class in games like Team Fortress 2. Playing as this class usually means the player is disguised as an enemy, stealthily killing other opponents. What the spy loses in firepower, he makes up in stealth.