Overclocking is basically running the CPU at a faster speed than it was intended - this is very much dependant on your motherboard, the CPU and your cooling ability (overclocked CPU's, because they run faster, also run hotter)
Running overlcoked may also shorten the life of a CPU (especially if its not cooled properly)
Realistically overclocking a CPU is like performance tuning a sports car, while it does increase the speed it's still pretty much in the realm of nerds who want to show off, as often the perfoamnce boost is minimal.
Personally if you want a faster CPU, I'd consider buying a new one - the 1.7-2.8ghz CPU type range is pretty cheap, providing your motherboard will take that faster CPU's