The climate control system in your car used to be pretty simple.
You could look at the red and blue colors and ballpark what you wanted via analog switchgear and estimation. Today, setting climate control in your car is more like using an app on your phone. Let's break it down.
Automatic. Most cars today have this mode and a digital setting for temperature, eschewing the old blue-to-red continuum of temperature of slide levers. Today's automatic mode is tied to a cabin temperature sensor and a slew of servos and vacuum motors that adjusts all the climate options to maintain the temp you want with low humidity and least amount of fan and its noise. If you're in a hurry, you can stop reading now. Automatic is all you really need to know. But if you like to roll your own, here's how:
Temperature. If you choose not to use automatic mode, you can still set temp and that will call up a certain temperature of air, blended from a combination of fresh air, warm air from the heater core and cool air from the A/C evaporator. Then you need to make some decisions what to do with that air.
Flow. These are the various levels or registers in your car. Defrost vents, instrument panel vents, and floor vents are the usual ones you have to choose from, depending where you like to get hit with hot or cold air, and whether your windshield is fogged or not.
Fan. Once you decide the temp and direction of air flow, you have to decide how hard you want to feel it. Fan speed handles that. On some late model luxury cars, there will also be a sub-selection of fan that can make the flow of air softer while still attaining the high volume of a fast fan speed.
Recirculate. This keeps recirculating the air in your car so less of it is exhausted out of the cabin. Useful when you want to cool a car down fast since it keeps chilling already chilled air. Also good when you're driving through Stinksville, but know that you need to move to recirc mode before you have a nasty smell inside the car, otherwise recirc just keeps it in there longer. Note that Max A/C also invokes recirculated airflow.
Defrost. There are usually two Defrost buttons.
The trapezoidal shaped one is for the front windshield and invokes a strong blast of defrosting air. There may also be a Max Defrost button that diverts all resources to defrost and from the rest of the cabin. The rectangular defrost button is for the rear window (also known as the backlight) and typically turns on a set of heating wires embedded in the glass.
Pro tip: Know that defrosting your windshield is not just a job for the heater. That concept is a holdover from the days when few cars had air conditioning. Now that they all do, front Defrost will often invoke the heater and air conditioner to create a mix of air that is both warm and dry to get the job done faster.
Zones. Modern cars frequently have dual zone climate control that separates the car into left and right halves, allowing you to sync them to the same temp or let them be adjusted separately. Unless the difference in set temp between the two zones is pretty dramatic, this sort of system can be a bit silly: Air doesn't know imaginary boundaries inside a car. Some cars also have a third zone for the rear seats or even quad zones for each seating zone in a luxury sedan.
Filtration. Your car may have a switchable filtration system, like Tesla's Bioweapon mode. These systems act on the incoming air to remove pollen or, as Tesla envisions it, Sarin and VX.
Scent. Some modern cars not only take stuff out of the air but also put scent into it. Recent Mercedes S Class and BMW Alpina B7s are among the cars I've driven with this feature, and wondered why. I'm somewhat more convinced of the usefulness of cabin air ionization found on some cars.