If you are new to electric RC models then this answers some of the basic questions about LIPO batteries
Quick summary:
C rating = the amount of current you an pull at any one time (related to the mAh rating)
mAh rating = the amount of energy sotred in the battery pack
2s, 3s, or 4s = how many cells are in a pack, each cell has 4.2 volts when charged
KV = this is for motors and tells you how many RPMs/volt you get under no load