|MadSci Network: Physics|
the explanation is quite simple. You have two different possibilities when you measure the specific heat - either you keep the pressure constant or the volume. In both cases you have the letter "C" and as an index you have either "v" if you keep the volume constant or "p" if you keep the pressure constant.
And - what is the bigger one? It is Cp. Why?
If you say "pressure constant!" then you allow the system to expand. And if it expands, it "works" - so the energy you put into the system is used for expansion (at least partially). In the case of constant volume you don't have this work - so Cv is a little bit smaller than Cp.
If you need more, read a book about physcial chemistry or so, Atkins for example.
Try the links in the MadSci Library for more information on Physics.