C rating on a battery is how many amps the battery will put out. the higher the amps the more torque your car has. 60c is usually the happy medium of discharge not too much and not too little.

It's the battery's ability to withstand current draw. The higher the number the higher the current draw the pack can provide.

If you're not racing a stock class anything over 30C would do you fine.

Btw, I'd personally steer clear of the duratrax and venom packs.