Cp is Process Capability Ratio, it tells us if a process is capable (at least 1 or greater). For six sigma the Cp value must be 2 or greater. It estimates what the process is capable of producing if the process mean were to be centered between the specification limits. It assumes process output is approximately normally distributed. Cpk is Process Capability Index, it estimates what the process is capable of producing, considering that the process mean may not be centered between the specification limits. Below is a video explaining the two.   You can download another video explaining what Cp/Cpk, Standard deviation and Mean are here: https://www.youtube.com/watch?v=TZRWWZuww4M