A simplified explanation of overclocking-
There are 2 ways to increase the speed of the CPU. Increase the multiplier, which requires an unlocked CPU. Or increase the Bus speed which requires an unlocked Motherboard. There are various ways to do these things, and various ways OEMs try to stop you.
When you increase the speed you create more heat. At some speed you will need to increase voltage for stability. That creates even more heat.
When testing for stabilty you need to monitor temperature to avoid damage. Different CPUs have different Voltage and temperature limits.
Cooling,Voltage , and speed. That's about it.