What is done to a car during a tune up?

Generally a tune up consists of spark plugs, wires, cap and rotor(DIS ignition has no cap/rotor), fuel filter, air filter, PCV valve. This is the general type. Others may define tune up another way and include other things. I define it this way. Going a bit further, there can be a timing belt replacement and further. But the general idea of tune up is the necessities I listed in the first line.