With appealing attributes such as low weight, high energy density, and ever greater discharge rates, Lithium-Polymer (LiPo) batteries have transformed all facets of RC. The emergence and continual improvement of these batteries has provided a significant performance boost for RC cars, boats, airplanes, and helicopters, while also paving the way for new vehicles such as multi-rotors. All of this electric goodness does not come without a cost. If not handled and utilized properly, LiPo batteries can quickly become damaged or even catch on fire.
When talking about a LiPo, the primary characteristics to understand are the battery's voltage and capacity. This is typically noted in a shorthand such as "4S-2200". In this example, "4S" denotes that the battery has four cells in series. The nominal voltage of each cell is 3.7 volts (4.2v fully-charged), so the total pack voltage is:

4 cells x 3.7v = 14.8v.

The second number denotes the capacity of the battery in milliamp-hours (mAh). A fully charged 2200mAh pack is rated to provide a current of 2200 milliamps (2.2 amps) for one hour before it is fully discharged. This capacity value is completely independent of how many cells are in series. In simple terms, the capacity value allows you to estimate how long a battery will provide useful power in a given application. In practical terms for RC use, the capacity rating is typically only helpful for rough comparisons of different batteries. i.e. a 2S-5000 battery will provide about double the run time of a 2S-2500 lipo in the same RC car.

WHERE IT CAN BE USED.
Another important aspect to understand about LiPo batteries is their discharge rate. This value allows you to determine how many amps the battery can output continuously without becoming damaged. Since many RC applications have high amperage requirements, the discharge rate of a battery can be a very significant factor in the overall performance of the vehicle. The discharge rate is expressed as a multiple of C. The internet can't seem to agree on what "C" really means. Whether it is coulombs of charge, the capacity of the battery, or the one-hour charge rate isn't really important for our purposes anyway. The end result is the same.

Going back to our example of a 4S-2200 battery, let's say it is rated for 20C discharge. To calculate the maximum discharge capability of the battery, we multiply 20 by the second number:

20 x 2200 = 44000 milliamps = 44 amps

The calculation tells us that this battery can safely be used in a system that is expected to continuously pull 44 amps or less. It also means that a fully-charged battery will last about three minutes at that current draw. Using the same logic, a 4S-2200 30C battery would be valid for applications requiring up to 66 amps of current, with a two minute duration.

For aircraft models, the common practice is to select a battery with a discharge rate that is at least equal to the maximum current draw of the aircraft.
It is rare to have a RC vehicle that pulls a continuous amperage value. The current draw will more likely be all over the map during a race or flight. For aircraft models, the common practice is to select a battery with a discharge rate that is at least equal to the maximum current draw of the aircraft (i.e. "full-throttle"). If you don't know the current draw of your aircraft, it can be measured or predicted.

With RC cars and boats, hard acceleration can cause huge short-term current spikes, which complicates things in terms of selecting a proper battery. When in doubt, choose a battery with a higher discharge rate. The performance of the vehicle will only benefit from it. The only downside is that you'll almost always pay more to get a higher discharge rate. Sometimes the batteries with higher discharge rates are slightly heavier as well.

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