It's often referred to as push and pull fans, the idea is simple enough,
you either push more air in, to create positive pressure,
or you pull air out, creating negative pressure.
If you remember that hot air rises, you can work out where exhaust should be, e.g. back, top of case, and intake should be near colder or more open areas of the room, e.g. front/side panels.
Each case design is different, typical cases with large fans installed eg 200mm, prefer positive pressure for quieter running, but they are often warmer as a result.
positive pressure
1. push/suck/inhale air into the case with multiple fans, and hot air will be exhausted in one place, usually at the top.
2. Pulling air in from a single large fan, and using small, slow exhaust fans creating pockets for air at the bottom of the case with ducting or solid objects.
negative pressure
1. Use a passive vent, and ducted areas to extract slow moving hot air, example is a PSU fan, or video card fan. Some cases also have exhaust fans near HDD drives to pull air from warm areas, without a large fan to pull air in.
2. Passive cooling cases, have airflow out only, several exhaust fans near the back or muffled fans to break fan noise or flow of air across the motherboard, cpu and gpu.
Most people choose quiet fans, 120mm at 900rom to 1200rpm is $3- $40, mostly cheap and effective, and can cool most PCs. smaller fans are noisier and take longer to move air around, which heats nearby devices. 200mm fans can have the same problem as small fans too, so it can be confusing.