Batteries in portable consumer devices such as a laptop, camcorder, cellular phone, etc., are typically made using either Nickel Cadmium (Ni-Cd), Nickel Metal Hydride (Ni-MH) or Lithium Ion (Li-Ion) battery cell chemistry. Each type of rechargeable battery chemistry has its own unique characteristics as follows:
1. Ni-Cd and Ni-MH:
The main difference between the two is that Ni-MH battery (the newer technology of the two) offers higher energy density than Ni-Cd. In other words, the capacity of a Ni-MH is approximately twice the capacity of its Ni-Cd counterpart. What this means is for you is increased run-time from the battery with no additional bulk or weight. Ni-MH also offers another major advantage: Ni-Cd batteries tend to suffer from what is called the "memory effect". Ni-MH batteries are less prone to develop this problem and thus require less maintenance and conditioning. Ni-MH batteries are also environmentally friendlier than Ni-Cd batteries since they do not contain heavy metals (which present serious landfill problems). Note: Not all devices can accept both Ni-Cd or Ni-MH batteries.
2. Lithium Ion:
Lithium-Ion (Li-Ion) has become the new standard for portable power in consumer devices. Li-Ion batterys produce the same energy as Ni-MH battery but weighs approximately 20%-35% less. This is can make a noticeable difference in devices such as cellular phones, camcorders or notebook computers where the battery makes up a significant portion of the total weight. Another reason Li-Ion batteries have become so popular is that they do not suffer from the "memory effect" at all. They are also environmentally friendly because they don't contain toxic materials such as Cadmium or Mercury. 
Is it Possible to Upgrade the Device's Battery to a newer Chemistry? Maybe. Ni-Cd, Ni-MH and Li-Ion are all fundamentally different from one another and cannot be substituted unless the device has been pre-configured from the factory to accept more than one type of battery chemistry.