MOA is the abbreviation for Minute of Angle, which measures 1/60 of 1 degree. If you want all the technical jargon and math that involves, this link explains it all:
http://en.wikipedia.org/wiki/Minute_of_angle
But when you're talking about rifle scopes, here's what you need to know:
Adjusting the angle of your scope by 1 MOA will change the point of impact by about 1 inch at a distance of 100 yards. So 4 MOA is roughly 4 inches over on an object viewed at 100 yards.
Usually, the windage and elevation adjustments on a scope will be about 1/4 MOA, or about a 0.25 inch change over 100 yards.
