In an attempt to help a fellow musician I wrote an article to discuss the major practical issues regarding amplifiers, cabs and power handling. First I'll tell you what were the answers to his questions:
So what does it mean when they say 'This amplifier is rated 200W at 4ohms'?
This means that the maximum power that the amplifier delivers is 200W and this happens at a load of 4ohms
What happens if you use a larger cab like 8ohms?
Trying to keep you away from the mathematics, in a nutshell, the amp will deliver less power and will also drive less the amp (so you're always safe not to blow the amp)
What happens if you use a smaller cab like 2ohms?
More current will flow, which will eventually drive the head even harder and will eventually blow the head. Note that the amp will still deliver LESS power. Yes dudes...don't think that by driving the amp harder you're getting more power :) You'll actually getting more REQUIRED POWER resulting in heat. The delivered power is always less...
To boil everything down:
1) You blow a cab if you use an amp rated more than the maximum power that the cab can handle
2) You blow a head if your total cab impedance is less than the impedance that the head can handle.
3) You get full amp power if you use the recommended impedance (impedance matching)
Now you can ask also questions like 'what's the difference between using two 8 ohm cab in parallel (which make a 4ohm total) and using 1 4ohm cab. This is now more of sound engineering which is a very nice subject. Ideally the bigger the speakers surface area the better, so you deliver bigger volumes of air which produces better sound...
In my next post I'll discuss the physics behind it.