MOA is the drop of the round to distance. 1 MOA= 1 inch per 100yrds
Actually not exactly true. 1 MOA is a 1" spread @100 yards of a pattern of shots fired. So it can be windage and/or elevation. You can have a 1 MOA and it potentially could be horizontal stringing only, left and/or right of the bulls eye.
1 MOA equals .5" @ 50 yards. Or .25" @ 25 yards.
Or 2" at 200 yards, or 3" at 300 yards......see the pattern here?
Here is a link to reference material:
http://nssf.org/video/facts/MOA.cfm