Overclocking is when you adjust settings above the factory defaults whether it be the CPU or Video card.
Example CPU: If your CPU runs at 3.0ghz and you adjust the clock speed to 3.5ghz.
Example Video: If your video card runs at 700Mhz/core 1400Mhz/Shader and 1200Mhz/Memory and you adjust it to 720Mhz/core 1440Mhz/Shader and 1250Mhz/Memory
There is more to overclocking than just raising the CPU frequency so if you plan on overclocking than read up and do some research. Overclocking the CPU involves a lot more research and time than Overclocking the Video card. http://www.tomshardware.com/forum/259903-29-overclocking-benchmarking-guideshttp://www.tomshardware.com/forum/258573-29-black-edition-overclock-guide
Also when you Overclock you want to keep the voltage and temps in the manufacturers safe ranges so you dont decrease the life expectancy of the CPU and video card.
Monitor temps with HWmonitor and stress test for stability and maximum temps with Prime95 and make sure your temps dont rise to high.
Post your System configuration so we can give you some more specific advise.