A Minute of angle or MOA is 1/60th of 1 degree. XP3Hunter pretty much hit it right on the nose for how you use minutes of angle off hand at different distances but technically 1 MOA is 1.0471996 inches at 100 yds. as you can see out to say 300 even 500 yds it works just fine reffering to it as 1in but at the farther distances where a bullet drops far more per distance distance traveled it is much more precise to use the actual value.
MOA can be used in 1 of 2 ways but both have to be considered while taking a long range shot. 1) The way XP3Hunter described which is used to determine the circle of accuracy you have at any given distance. EX: if you are shooting 1/2in groups (series of 3 or more shots) at 100 yds, generally you can expect to be in a 1in group and 200yds, 1.5 in groups at 300yds ect. 2) it is also used to describe the elevation (bullet drop due to gravity) and windage (bullets left or right movement due to wind deflection). If your bullet drops 2in from your POA(point of aim) at 200 yds its elevation drop is 1 MOA which would need to be compansated for either via holdover, dialing in, or some other form of BDC(bullet drop compensation) so you match your POA to your POI(point of impact)
Sorry for all the abbreviations but i figure you will run into all of them here on the site. I hope this helps :D
