NiMH (Nickel-Metal Hydride) rechargeable batteries are newer than NiCd (Nickel-Cadmium) batteries. NiMH has up to 30% to 50% more run time per volume than NiCd; i.e. it has a higher energy density and delivers about twice the capacity. NiMH are also more environmentally friendly because they do not have heavy metals. NiMH batteries also can be charged without suffering from the "memory effect".
Battery performance is improved by doing a charge pulse and then a discharge pulse or with a Delta V terminating charger. This is known as "burp" or "reverse load" charging. A NiMH battery may be damaged by extensive over-charging (on a charger for more than 24 hours). NiMH batteries are more stable while being charged and have less voltage drop when full capacity has been reached but NiMH batteries don't respond well to being stored in a completely discharged state. NiMH batteries can last up to 1000 recharges.
sources: various sites from around the web. I wanted to learn about NiMH batteries after purchasing a set for my digital camera.