NiMH is an abbreviation that stands for Nickel Metal Hydride, which is actually what makes up the major components of the batteries or the materials that are used for battery parts. There are also some other types of batteries, such as Li-ion, NiCd, Lead-Acid, and so more and so forth.
While on the other hand, the term mAH is an abbreviation that stands for a milli-ampere hour, which also means the rating for the amount of current that is expected to be drawn for a given period of time. Just a single type of battery may come with mAH capacities, which vary.
The typical mAH values or ratings for AA batteries are usually 1000mAH, 2000mAH, and 2400mAH. What NiMH would dictate is the characteristics of a particular battery, while on the other hand, mAH would determine how long a battery will last. NiMH batteries are expected to be charged with a NiMH battery charger.