Lithium Polymer (Lipo) battery technology has been perhaps the best leap of technology in the RC world. The relatively high energy density and absurdly high continuous current capability has meant that quadcopters have become a viable hobby and electric RC cars can now outperform their Nitro cousins without most of the fuss.
Voltage as measured in volts (v)
A single lipo cells has a maximum charge voltage of 4.2v which is usually reported as 3.7v for real-world purposes. Voltage can be increased by connecting these cells in series (s for series) and is reported as 1s (1 cell in series), 2s (2 cells in series) 3s (3 cells in series and so on). Voltage is additive when cells are placed in series i.e. 1s = 3.7v, 2s = 7.4v, 3s = 11.1v and so on.
Capacity as measured in milli Amp hours (mAh)
Lipo cells can vary in capacity, these are measured in mah and signify the amount of 'fuel' the lipo battery can output before it is depleted. All else being equal the larger the capacity they larger the physical size and mass (weight).
C - Rating
Where voltage and capacity are simple physical parameters, C-rating is slightly more complex. It is closely tied to capacity and has no relationship to voltage. In short the C rating signifies the maximum continuous output that the battery can maintain. Strictly speaking the C value multiplied by the capacity is the maximum continuous current output which we measure in Amps. The best way to describe this is to give examples:
RC car technology has changed a lot over the years and one of the biggest changes have been batteries. I started with Sanyo SCRC NiCd pack, left the hobby before NiMH came out and came back when LiPo was full swing. Along with brushless power systems, LiPo batteries have revolutionised all RC vehicles - more burst power, higher energy density and multiple cell options. LiPo batteries need a lot of care, more than than a set of nichrome wires that hook your battery pack directly to a car battery anyway. The technology you need to properly charge and care for Lipo (and LiIon batteries) is a balance charger which allows the charger to charge each individual cell at a time rather than all together in series. This is important to not only extend the life and performance of your battery but to minimise the risk of the Lipo pack causing damage to your surroundings.
The right tool for the job
Like everything else to do with RC Cars, LiPo balance chargers are available in any number of models with different features, different interfaces and different quality levels. However unlike, motors, wheels, servos, escs and any other RC car parts they don't wear out and typically the charger you use will be with you for a long time so it is definitely worth having one that does the job properly. With that said you don't need to spend a lot to get a good charger that you can trust with your safety and life of batteries. This is in contrast to charger that come with RTR RC Car kits which at best are inaccurate and at worst are downright dangerous.