Before electronically-controlled vehicles, a tune-up included maintenance adjustments to the engine and replacement of the spark plugs and distributor points, the distributor cap and rotor, and the fuel and air filters. As of 2014, many newer vehicles have on-board computers that automatically perform maintenance adjustments, so tune-ups aren't necessary.
Tune-ups are operations performed primarily on a vehicle's engine system that are done yearly as preventative maintenance, not to fix any problems with a running car. If a tune-up caused a noticeable improvement in the performance of a car, then the owner waited too long for the service to be done. Repair shops vary in what is included in a tune-up, so manufacturers don't include a tune-up in the maintenance schedule. Instead, service items are individually listed to be done at a specified mileage outlined in the owners' manual. These scheduled services might include tune-up procedures, but they also specify other repairs to be done throughout the vehicle.
The term "tune-up" came to be because early vehicles used simple ignition systems that provided one ignition coil per spark plug. For better performance, each coil needed adjustments so that they all sparked at the same intensity. This adjustment caused them to produce a buzzing noise that were in tune with each other, hence the term "tune-up."