NiMH - Most common type of battery, pretty safe to use
LiPo - offers more amperage than NiMH, so you can have less voltage for the same rate of fire, but you need to know what the hell your doing when using LiPo as they vent violently when broken, and they break under different conditions than NiMH.
It's important to know what kind of battery you have when you pick your charger.
-Voltage: increases ROF and trigger response. Too much voltage means too much speed which means gun problems. 8.4v is the standard for large batteries (over 2200mah), and 9.6v is the standard for mini batteries (1200-1600mah)
-Mah: The amount of gas in your battery, the more gas there is, the bigger the battery is, and the higher the discharge rate
-discharge rate: how much amperage your battery puts out, physically larger cells have a higher discharge rate. IE a 9.6v mini battery will have the same ROF as an 8.4v large battery.
Think of the electrical flow as a river, amperage is the amount of water in the river (little stream or big river), voltage is how fast that river is flowing, and resistance is the rocks slowing down the water.
So a battery with high amperage and low voltage (7.4v 20C 1600mah LiPo) would give you the same rate of fire as a battery with high voltage and low amperage (9.6v 1500mah NiMH).