Batteries can seem overly complicated and there are a number of options from Nicd, Nimh, LIPO and LIFE.
Personally I would recommend that you discuss your options with the local model shop or club and would also divert you to the electric board on the BRCA website,
www.brca.org this lists a number of batteries which are all deemed as safe to use by the sports governing body.
As a rough guide you will require a battery with a nominal voltage of 7.2volts or 7.4volts for a 1/10 scale battery. This would equate to either 6 x 1.2volt nimh or nicd batteries or a 2s LIPO battery. The S on lipo seems to relate to the nominal voltage of a battery in this case it is 3.7volts, so 2s is 2 x 3.7volts.
The mah relates to the batteries capacity, which is basically how big the fuel tank is, so the higher the mah the longer the car will run for. It may also make the car a little bit faster than if using a lower capacity battery.
Please also make sure that you use the appropriate charger for the type of battery you purchase.