The term "tune up" is subject to interpretation these days.
In decades past, a "tune up" was maintenance for your engine's performance. A mechanic would replace your spark plugs, ignition wires, distributor cap, rotor, points, set your timing, adjust your carburetor...etc...etc...
These days, since vehicles have evolved so much, a "Tune Up" is limited to replacing spark plugs, checking your throttle body for contamination, checking your air filter...and that's about it. You could get into plug wires and a cap and rotor if you have them still.
Finally, some folks refer to any mileage maintenance service (like a 30,000 or 45,000 mile service) as a "tune up". This would include basic maintenance like oil changes, etc...
When does your car need a tune up? It's best to refer to your owners manual and maintenance guide for the appropriate intervals of all maintenance service.
Go to that link to find a way to obtain a manual if you do not already have one handy.