Current RC cars almost exclusively use LiPo (Lithium Polymer) batteries, aside from small scale or other niche classes. LiPo cells (individual batteries) provide 3.7 volts each, and are then combined to produce various voltages.
This one is pretty straightforward - a battery's capacity, how much charge it can hold, is shown in milliamp-hours, mAh. More mAh, more capacity.
Batteries with high capacity are common for stock racing, where you need all the punch you can get for the entire race, while modified classes often use smaller batteries because they get enough power from the more powerful motor already and they're trying to save weight.
Battery S rating
The "S" number on a battery indicates how many cells are in series, that is, how many are connected together to generate a certain voltage level. Lithium Polymer batteries (LiPo) have a voltage of 3.7V each, so each "S" gives you another 3.7V. Many cars use 2S which gives 7.4V (2 x 3.7 = 7.4), very close to the voltage traditional Nickel Cadmium (NiCd) packs that were used back in the day.
Some 1/8th scale cars and bashers use more cells in series these days, with 6S being common in bashing, giving a whopping 22.2V of voltage.
In niche applications, you may also see a "P" in the rating for a battery - this indicates there are cells wired in parallel which gives more capacity but keeps the voltage the same.
Whatever battery you go with, make sure your ESC is rated to handle it, and watch your motor closely as well - they can both overheat!
Battery C rating
The "C" rating on a battery stands for its "discharge rate," that is, how quickly it wants to discharge (deliver its power to your car.)
Higher C rating
- More punch when accelerating
- Higher discharge load