S is the number of cells in a pack. So 6s means there are 6 cells. To work out the fully charged voltage multiply this by 4.2V.

C is a multiplier that tells you charge and discharge rates. 50C means that the pack can discharge 50 times whatever it's capacity is. So a 4000mAh pack with 50C discharge could in theory give you 200A. Charging is generally in the order of 1 to 3C.

Jamie is correct that I use a quad reaktor charger. Excellent charger doing everything you would need with lipo batteries. Important thing being to balance charge keeping all the cells the same voltage and if you aren't going to use them for any length of time to put them into storage voltage which is about 60 to 70% capacity from memory.