There are several ways the 'C' figures into lipos - prob the most important is in figuring current the lipo will deliver. Example if you have a 25C 5000mah lipo, this means it is supposed to be capable of putting out 5000/1000 x 25 = 125amps continuous current. There is another C rating which is the burst rating and its usually only good for 15 - 30seconds. Its higher than the continuous C rating. For the example, that batt may have a 30C rating meaining for short a period, that lipo could put out 150amps - (5000/1000 x 30 = 150amps).
The C is also used in charging as a 1C charge which most lipos are charged at, again, its the lipo mah divided by 1000, again, example - 5000/1000 = 5amp charge.