the C rating is how much current you can discharge. 1C is the capacity of the pack in amp hours. Since a milliamp hour or mah is 1/1000th on a amp move the decimal place over 3 spots or divide by the mah capacity by 1000 to get your 1C. So the 1C rating of a 4000mah pack is 4 amps.

The 20C and 30C refer to the maximum rate of discharge. If the pack is a 20C pack it can discharge at 20X the 1C rate or 80 amps. The 30C pack is 30 X or 120 amps. So generally speaking a higher C pack will do two things, 1) let you discharge at a higher rate, and 2) have a higher voltage at the same rate. So this means you should have more punch and speed with a higher C pack. If you are running stock motors the difference is minimal, but as the motors get faster, the higher C packs will make a difference.

John