On-board diagnostics are the systems embedded in cars that allow problems to be diagnosed electronically. Beginning with simple devices as far back as 1969 that detected problems and lit an "idiot light" alerting of a need to get the car serviced, they grew in sophistication over the years, eventually being mandated by various governments and having their protocols for giving details of automotive problems standardized. To the end-user, though, they mostly appear as the same old "idiot light" indicating a need for service with a frustrating lack of detail about just what is wrong (which is expected to be determined by the dealer or garage when you bring the car in), but devices are available that can be hooked up to the car's standard interface adapter to get complete details for satisfying curiosity or aiding in do-it-yourself repairs.
The current standard is known as OBD II.
Security is not very good on these devices, and there are reports of OBD systems being hacked for the purpose of auto theft or vandalism, or invasion of privacy by keeping track of everything a driver does. Vehicle fleets are sometimes monitored by their corporate owners using data from the OBD systems. Systems that keep an even greater amount of detail on car activity are known as automotive black boxes.