Fastbacks, notchbacks and squarebacks - how they produce lift.. and how to reduce it!*
I am aware that lots of people here are unconcerned with aero lift on road cars, but having given a car real-world downforce at normal road speeds (eg 100 km/h - 60 mph), I can vouch for how important it is to stability and good driveability.
(And of course, in general, a car with zero lift has lower drag than one with lift - and 99 percent of unmodified road cars have lift.)
*Note that I am talking about modern cars that have attached flow on their upper surfaces, some small separation bubbles excluded.