This is the act of secretly watching a person or group to gather information, often for political purposes. Spies typically disguise themselves so that they might sneak around. Espionage is also frequently tied to assassination plots.
Typically, a spy either blends in with their surrounding or hides, or places equipment in a room place to spy on people without actually being there (equipment suck as cameras and microphones). By doing this, a spy may learn of information which would otherwise be unattainable.
MM! Hidden Cam special
A lesser form of espionage perhaps seen more frequently in anime and manga is peeping on someone (usually, a girl, or group of girls). This often takes place in changing rooms/locker rooms, hot springs and swimming pools. More often than not, the assailant(s) is caught, and beaten to the point of near death by the victim(s). Some franchises feature scenes like this almost constantly, like Sora no Otoshimono, which finds Tomoki often peeping on the numerous girls in the series, while other series, such as MM!, even offer bonus peeping scenes as extras with DVD/Blu-ray purchases. These typically have no real impact on the story or characters and simply serve as a form of fanservice.