[1][2][3] It is the automated equivalent of the paper toll tickets that were written and timed by operators for long-distance calls in a manual telephone exchange.
For law enforcement, call detail records provide a wealth of information that can help to identify suspects, in that they can reveal details as to an individual's relationships with associates, communication and behavior patterns, and even location data that can establish the whereabouts of an individual during the entirety of the call.
The U.S. Supreme Court has held the records of numbers called are not protected by the Fourth Amendment to the Constitution of the United States because the caller "voluntarily conveyed numerical information to the telephone company.
In June 2013, a top secret order of the United States Foreign Intelligence Surveillance Court was leaked to the public.
That order referenced and defined call detail records as follows: "IT IS HEREBY ORDERED that, the Custodian of Records shall produce to the National Security Agency (NSA) upon service of this Order, and continue production on an ongoing daily basis thereafter for the duration of this Order, unless ordered by the Court, an electronic copy of the following tangible things: all call detail records or "telephony metadata" created by Verizon for communications (i) between the United States and abroad; or (ii) wholly within the United States, including local telephone calls.... "Telephony metadata includes comprehensive communications routing information, including but not limited to session identifying information (e.g., originating and terminating telephone number, International Mobile Subscriber Identity (IMSI) number, International Mobile station Equipment Identity (IMEI) number, etc.