What is Overclocking and how it is achieved ?

Overclocking is just running CPU faster than what is mentioned or certified from the factory. Many a time the overclocking helps quite a bit and it does not cost anything to do it. Sometimes Overclocking is safe but sometimes it can be dangerous too that can damage hardware.

For example, Q6600 is Intel CPU certified to run at 2.4GHz per core. But it can run at 3GHz per core provided you have a good motherboard and a good cooling solution.  If the temperature is cool and the CPU runs faster.
If it is running it at 80C or higher and putting in more than 1.5 volts into it., Would make it run hot and burn up the CPU.

There are some videos showing people using liquid nitrogen to cool the CPU so that it will run at 4Ghz or even more just to see how far it can go without heat being an issue.

But if you keep the voltages low from its limit and the processor in its normal operating range there is no damage to the CPU and it will last for at least 8 years or more. Usually by 8 years you might have already upgraded anyways.

These days you might have heard about turbo boost. What that means is the CPU will turn off 2 or more cores and it will overclock itself to the two active cores making the programs that can only use two processors work faster by providing more clock speed. The reason it turns off the 2 other cores is to reduce heat which is bad for CPU's
There is two features which can help keep a processor cool even when its overclocked and they are called Cool N Quiet which is for AMD and Speed Step which is Intel. But both do the same thing. They lower the clock speed when there are no demanding applications running. By lowering the CPU clock speed the less heat is made. When there is a huge CPU demand the clock speed will go to the max speed the CPU is rated at or more if it’s overclocked.

There is some super clocked video cards that are overclocked straight from the manufacture. Sometimes the GPU speed is 10 to 15 MHz or slightly more. But it’s not very much overclock from stock speeds and the memory is overclocked slightly faster than stock. But I really doubt it helps all that much. If you push it too far you can ruin the GPU and or it will cause artifacts or lockup or quit working.