Lithium Polymer batteries or LiPo provide much more energy per unit of weight than other types of batteries, making it the best choice for electric planes. Their only downfall is they are very sensitive and if not charged properly can actually explode causing fires in homes and cars. In recent years they have gotten much better and safer but still require care for use. If charged too fast or overcharged, the cells will balloon and heat up. Each cell has to be charged evenly using a balanced charger that monitors the voltage in each cell. They can also be damaged if the voltage drops below 3 volts per cell.
Today after a fairly long flight I took the plane in and was working on it and left everything plugged in. The Electronic Speed Controller (ESC) has an auto shut off that will turn the motor off before the voltage drops too low while leaving the rest of the plane on so you can still control it if your battery dies mid flight. After a few hours with the battery plugged in, I came back to find the Xbee was off and the lights on the APM board were dim. When I checked the LiPo it was warm and had already started bulging. I pulled it out and checked the voltage with a multimeter and found the 3 cell battery (that should not be below 9 volts) was at 3 volts. I threw it out and counted it as a cheap lesson learned about LiPos.