Okay. So, I had a chance to go talk to the manager at my local Guitar Center. He was very patient with all my questions and had alot of insightful answers. I’ll try and distill it down. As @Jazzbass19 said, it can turn into a bit of a rabbit hole. Please keep in mind I am not a professional and this is probably over simplified.
The wattage stuff is fairly self explanatory but I thought ohms would be easier to explain if I started with wattage to create context.
Wattage - The amount of power your amp head can put out.
Ohms - The resistance your cabinets have to that power.
Resistance generates heat. You can think of the Ohms rating on your head as how much heat your head is able to disperse before something starts to melt down, blow out, or catch on fire.
Head - If you have an 800 watt head you are able to put out 800 watts. But the wattage of your cabinets will determine how much of that 800 watts you use.
Cabinet - If you have an 800 watt head but only have a 400 watt cabinet, you are only putting out 400 watts. To get 800 watts you will need to add 400 more watts of cabinets. If you have 1200 watts of cabinets you are still only producing 800 watts because that’s the maximum for the amp head.
Back to ohms…
Here’s the progression you need to know; one 4 ohm = two 8 ohm = four 16 ohm
If the amp head is rated up to 4 ohms, it can handle…
one 4 ohm cabinet or
two 8 ohm cabinets or
four 16 ohm cabinets or
one 8 ohm cabinet and two 16 ohm cabinets
If the amp head is rated up to 4 ohms, do NOT exceed the 4 ohms or the amp head can be damaged.
Bringing it all together…
Without a physics background the ohm ratings seem counter intuitive and can be confusing. It’s really just about what can plug into whatever else. If you’re putting together a stack, it is much more intuitive if you go to the store and look at it.
Ultimately, I didn’t really need to know any of the ohm stuff and 90% of musicians probably don’t either. But I didn’t know this till I started doing research.