Sven - MOA is simply a convenient method of describing or assessing the accuracy of a rifle. At 100 yds you could just as easily say that the rifle is guaranteed to make 3 shot groups of 1 inch or less. But, what makes the MOA method better is that it can be translated to different yardages if you know the MOA accuracy at, say, 100 yds. At 200 yds, that same rifle should keep groups in a 2 inch circle, 4 inches at 400 yds, etc. This is not to say that accuracy is a linear concept due to the many variables that enter into it: bullet stability, wind, and the rifle itself. With some rifles which are accurate at up to 300 yds, the accuracy can fall off after that whereas other rifles will hold their accuracy out to 1000 or more yds. The subtle differences in the rifles make the difference.
A mathematical note: There are several methods to determine the actual size of a MOA at any distance. You can use trig functions, but the simplest is to take the circumference of a circle whose radius is the target distance and divide by the numbr of minutes in a circle as BigG noted. 100 yds * 36 inches/yd *2 * PI / 360 deg * 60 min/deg = 3600 * 2 * 3.14159 / 21600 = 1.05 inches in a MOA at 100 yds.