Overclocking is increasing a chips speed by increasing the amount of power it receives. It can be harmful if you do not properly cool the component, this will cause the computer to become unstable. If properly cooled, it can be beneficial.
Overclocking is increasing the clock rate of a CPU. The CPU is at the heart of a computer. It's what processes instructions. Many due it to get just that slight edge in performance mostly for games. Others do it as a hobby for bragging rights.
Overclock and bump the voltage too much and you'll fry your CPU. Just look at this twit. LMAO