The Batarang used in Batman was a foldable metal bat attached to a line and was used to ensnare an enemy's legs and manually drag them back. Batman Returns also featured a computerized version that could be programmed to fly after specific targets.
The 2005 film Batman Begins showed them as a simple bat-shaped shuriken, used mainly for distraction rather than as weapons, fitting in with that film's depiction of Batman's ninja training.
While the The Dark Knight uses the batarang in its promotional posters, it is not thrown in the film. As part of Lucius Fox's improvements on the Batsuit, he adds blades on the suit that shoot out of Batman's arm, which are similar to the batarang. To gain leverage over his fight with the Joker, Batman shoots these blades at him, further disfiguring his already scarred face. His true batarangs are only seen once during the film; when Bruce Wayne puts away his Batsuit after deciding to turn himself in to the police, he takes a minute to pick up and stare at one of his batarangs, and then proceeds to put it away with the rest of his gear.