By core and memory clock, I'll assume you're talking about CPU internal core clock speed, and the FSB (front side bus) speed. CPUs have 2 clock speeds at which they operate, an internal core clock and an external FSB clock speed. The FSB speed is the clock speed of the bus that connects the CPU to the RAM. This is usually 200MHz for most system but varies. However, to enable CPUs to operate at speeds much higher than this without forcing other components to also increase is speed; CPUs have an internal core clock which is the FSB speed multipled by a multiplier. E.g. if the FSB speed was 200MHz and the multiplier was x10 then the CPUs internal clock speed would by 2000MHz or 2GHz. The RAM usually works at the clock speed of the FSB, however you may see DDR400 labelled RAM modules when they actually operate at 200MHz.This is because they are DDR sticks, which double throughput and therefore effectively work at 400MHz. The memory clock does not have to be the same speed as the FSB, as you can set FSB:RAM ratios so you can alter the relationship.
I think he may mean for graphics cards. For example when you use coolbits to overclock an Nvidia card you have the option to overclock the core clock and the memory. Unfortunately i don't know much about this either. I've overclocked graphics cards in the past, but typically stopped after i started buying good cards so that i can get the most life out of them.
