As msween00 stated, mAh is a "rating" manufacturers assign to their batteries to give customers an idea how long that battery should last between charges in normal usage. 100 mAh = 1 hour of use. Therefore a 1600 mAh battery should be expected to last 16 hours, a 2000 mAh 20 hours.
This rating will help differentiate the batteries from an individual manufacturer's lineup, but perhaps not so much from one manufacturer to another. Some manufacturers inflate the mAh rating to make their battery appear to be superior, but in reality this may not be so when put into use. I wouldn't put much emphysis on the mAh ratings from one manufacturer to another, as the rating is made by the manufacture and there is no standardization to determine how they come up with the number
I suspect AW and Panasonic's mAh ratings to be more accurate than those of other brands.
I've seen bar graph tests of the AW IMR 1600 vs the AW IMR 2000 mah batteries. The 2000 mAh battery will last longer by nearly 4 more additional hours, but the 1600 mAh battery held its useful voltage better up to the point of it being discharged (in effect, with less voltage drop off as the battery is being used). So going by this, the 2000 mAh will last longer, but the 1600 mAh will have less of a voltage drop off for as long as it is in use.
