C rating can be thought of as the lipos discharge capability. If you have a 20C 5000mah its discharge capability is 20 x 5000/1000 = 100amps. The same capacity but 30C would have 30 x 5 = 150amps. So, there is a diff. It is indirectky influenced by the mah or capacity as you can see with how the discharge capability is calculated. A 40C 3400mah would have 136amps and have more discharge capability than a 20C 5000mah which would only be 100amps.

Where does it make a diff? It depends on the setuo you;re running and how many amps it needs. If you're running a heavy rc like a Flux, the higher C rating does make a diff you can see. If you're running a 5lb onroad car, it is prob not going to make a visible diff. They other way the C rating shows up is in lipo temp - if your batt is having to supply 80-90 of its C capability a lot of the time, it will run hot which is not what you want for long life. If you size teh C rating so the setup is using a lower percentage of its capability, it will run cooler and have a longer life.

There are some good links here, read teh one from the Heli site, it will help with understanding - http://www.rctech.net/forum/radio-el...o-newbies.html