For mobile telecommunications, the Charging Data Record (CDR) is, in 3GPP parlance, a formatted collection of information about a chargeable telecommunication event (making a phone call, using the Internet from your mobile device).
CDRs are used for user billing: a telecom provider transfers them from time to time in order to send bills to their users.
CDRs are sent in GTP' messages, or saved in files and fetched with FTP protocol.
A separate CDR is generated for each party to be charged.
Entries on CDRs use a {category, usage} syntax.