The Bat-signal is a spotlight marked with Batman's insignia on top of the Gotham City Police Department Headquarters in Gotham City. It is used by the GCPD to call upon Batman should they ever need him, however, it is also sometimes used as a reminder for criminals and innocents alike of Batman's presence in the city and has become somewhat of an icon for Gotham City.
As part of their working relationship, Batman (Bruce Wayne) provided the signal to then-newly promoted Police Commissioner James Gordon as a symbol of trust and a way to communicate without skulduggery.
Through the years various people have accessed the signal, each for his or her own reasons. Even criminals have used it to summon Batman's protection - although in doing so they turn the bat-symbol upside down, imitating the international symbol for distress.
The presence or absence of the signal usually serves as an indication of the Bat Family's current status with the GCPD.
The Bat-signal first appeared in Detective Comics #60 (February, 1942)
To protect the Gotham City Police Department from the legal issues of summoning a costumed vigilante, no representative of the police was allowed to access the signal. Instead the G.C.P.D. employed a civilian aide to handle the light.
In addition to the spotlight, the Bat Family also possess miniature high-powered infrared signals to signal each other around Gotham. These signals can only be seen in the infrared spectrum to ensure secrecy from the public.
In addition to the primary signal on the GCPD Headquarters roof, the GCPD possesses a portable signal fixed on top of a police van and a spare bat-signal for emergency uses. The spare is a more sophisticated laser signal which paints a green bat symbol in the clouds. This signal was donated by Kord Industries, however, the signal was said to have been unused because the city council deemed it to be an "inappropriate gift."
Upon Batman's return from his one year of self-imposed exile following the Infinite Crisis, the Bat-signal was activated once again. The familiar sight of the Bat symbol in the sky prompted cheers from most of the citizens of Gotham.