Nickel Cadmium (Ni-Cd)
Nickel Cadmium (Ni-Cd) batteries were the standard technology for many years, but today they are old technology and new laptops don't use them anymore. They are heavy and very prone to the "memory effect". When recharging a NiCd battery that has not been fully discharged, it "remembers" the old charge and continues there the next time you use it. The memory effect is caused by crystallization of the battery's substances and can permanently reduce your battery's lifetime, even make it useless. To avoid it, you should completely discharge the battery and then fully recharge it again at least once every few weeks. As this battery contains cadmium, a toxic material, it should always be recycled or disposed of properly.
Nickel Metal Hydride (Ni-MH)
Nickel Metal Hydride (Ni-MH) batteries are the cadmium-free replacement for NiCad. They are less affected by the memory effect than NiCd and thus require less maintenance and conditioning. However, they have problems at very high or low room temperatures which might actually cause them to catch fire. And even though they use less hazardous materials (i.e., they do not contain heavy metals), they cannot be fully recycled (yet). Another main difference between NiCad and NiMH is that NiMH battery offers higher energy density than NiCads. In other words, the capacity of a NiMH is approximately twice the capacity of its NiCad counterpart. What this means for you is increased run-time from the battery with no additional bulk or weight.
Lithium Ion (Li-ion)
Lithium Ion (Li-ion) are the new standard for portable power. Li-ion batteries produce the same energy as NiMH but weighs approximately 20%-35% less. They do not suffer significantly from the memory effect unlike their NiMH and Ni-Cd counterparts. Also, their substances are less hazardous. Because lithium ignites very easily, they require special handling.