Nickel metal hydride are rechargeable batteries used like nicads. Compared to nicads they have higher capacity, but lower max current delivery, quicker self discharge (typically 30% discharge per month), and voltage output falls as they near discharge. They also require NiMH chargers. Unlike nicads they don't contain the toxic heavy metal cadmium.
Lower max current delivery makes them a lower power choice for cordless tools. Faster self discharge makes them an inferior choice for items that will sit for weeks or months unused.
High capacity makes them the prime choice for high consumption appliances where batteries get replaced frequently.
Low discharge NiMH are now common. These are often sold as pre-charged. These don't suffer higher self discharge than NiCds.