The Bat-Signal is a specially modified Klieg searchlight with Batman's stylized symbol attached to the glass used by the Gotham City Police Department, most notably the Major Crimes Unit, to call Batman on matters of major criminal activity in Gotham City.
Batman Begins Edit
Sergeant James Gordon finds the mobster Carmine Falcone strapped on to a searchlight in the docks of Gotham City, for the Gotham Police force to arrest him, left by Batman. Gordon then notices that Falcone's shadow is projected into the clouds of the night sky, similar to the silhouette of a bat. At the end of the film, the Bat-Signal appears, as a searchlight that projects the shape of a bat, installed atop police headquarters as a means to contact Batman.
The Dark Knight Edit
As in Frank Miller's Batman: The Dark Knight Returns, Gordon uses the Bat-Signal to remind Gotham of Batman's presence. The signal proves to be very effective, with drug dealers and criminals becoming apprehensive at its very appearance. At the end of the film, after reluctantly agreeing to let Batman take the blame for the murders committed by Harvey Dent in order to preserve Dent's image as Gotham's hero, Gordon hesitantly destroys the signal using an axe in front of various members of the police force and the press.
The Dark Knight Rises Edit
The rusted remains of the destroyed Bat-Signal are still atop police headquarters. However, at the end of the film, with Batman declared dead, Gordon sees a restored Bat-Signal, providing hope that Batman has survived. (The signal itself is never used once in the film, however, making it the only live-action film about Batman where this occurs.)