What does it involve, besides the obvious. Thanks
Depends on what you get into. You could install HVAC equipment on new construction, retrofit older buildings with new HVAC equipment, be a HVAC service mechanic. It really should be HVACR where R is refrigeration. It technically is the same theory just at a lower space temp. In that case you might specialize in store refrigeration instead of comfort cooling/heating. Once again... Install, retrofit, service mechanic. There is always going to be some cross over between the different areas but depending on what type of company you end up working for you will more than likely end up working specifically in that area of the trade. The trade for the lack of a better comparison is solely based off the laws of thermodynamics; take heat energy from one area and put it in another area.
When it comes to HVACR consider it this way; anything that moves, cools, or heats a 'medium' for the express purpose of conditioning the temperature/humidity of a space or substance you will likely deal with at some point in time. This includes but isn't limited to air conditioners, chillers, boilers, heat pumps, hydronic systems and piping systems, pumps, heat exchangers, fan coil units, exhaust fans, humidifiers, refrigerators, freezers, cooling towers, cascading systems, brine systems, and did I happen to mention air conditioners.
Personally I started out in comfort cooling as a service mechanic. I saw lots of routine maintenance with very little service or trouble calls. It was pretty limited to heating and air conditioning. As I gained more experience and schooling I was allowed to go on more service calls. The more service calls I went on, the more I saw different types of systems. I think service calls are more the way to go, they keep your mind focused on the work trying to figure the problem out rather than just going through the motions of installing or performing routine maintenance. It makes the day go by a lot faster it seems, at least for me it does.