Ni-Cad (Nickel Cadmium) battery.
These batteries are reasonably priced but are very damaging to the environment. They also need to be completely discharged and cool to be able to accept a full charge.
Ni-MH (Nickel Metal Hydride) battery.
With this battery technology it was possible to increased the amperage to 3.0Ah, they are also a better option environmentally than Ni-Cad. They can be fully charged whether they are warm or partially discharged.
Li-ion or Lithium-ion batteries.
Lighter weight, quicker charging and with more run time than an equal voltage Ni-Cad or Ni-MH battery. These batteries have a chip that communicates with the charger for optimum charging. Lithium-ion batteries also offer much improved charge retention over long periods of storage. Li-ion batteries do not have to be fully discharged before charging in fact they can be charged at any time with no ill effect.