There are an increasing number of rechargeable batteries, for various devices -- and even batteries that can be charged by solar power. Here are some terms you should know.
Volts, or Voltage -- is also called "electrical potential difference" or "electrical tension" is the electrical potential difference between two points. It is the potential to cause current. A 12-volt battery has the 12-volts of potential energy. Note, however, that a 120-volt household outlet has potential -- but nothing happens until you plug something in, turn it on -- and complete the circuit, so an electrical current can flow.
Current - is the flow of electrons.
Amp or Ampere -- is an international standard unit of electrical current. It is the number of electrons passing a point in a given amount of time. More specifically, 1 amp = 6.241 x 10^18 electrons / second. 1 amp = 1 couloumb / second. At 120 volts, an average toaster may use 16 amps; lightbulbs = 500 mAmps (milli-amps); household circuit breakers are typically set to go off at around 15-20 amps.
mAh - Milli-Amp-hour -- is a init of electric charge. Today, a rechargeable AA-sized NMh battery might have 2800 mAh, while a smaller "AAA" battery may only have 1000 mAh. An alkaline 9-volt battery might have about 5000 mAh of energy stored. Some newer lithium-ion batteries store even more power than alkaline batteries! Lithium-ion batteries are only available for about 3-volts and higher (the reason why NMH batteries are used for AA or AAA batteries).