MOA means minute of an angle, ie 1/60 degree, roughly it's 1 inch (1.045 more precisely) per each 100 yards, or 29 mm per 100 m. It shows the amount of dispersion of the rifle/ammo system. You can visualize it with the light, the lesser dispersion, the tighter the "beam" is.
Conversion factor is Tan(1/60), or 2.9e-4, or 0.00029. you multiply it with the distance to get the amount of dispersion.
sub moa means that the rifle/ammo combo puts bullets into the circle that is no bigger than 1 inch at 100 yards. All the fractional moas mean the same, for example DSR-1 is a 1/5 moa rifle, with equal ammo it means the group size will be 1/5 inch at 100 yards, basicly a ragged hole.
But be warned, accuracy at long range depends a lot from the ammo, if one rifle/ammo combo shoots 1 moa at 100 yards it doesn't mean it shoots 6 inch group at 600 yards.
For example I shot 10-15cm group at 300 m (330 yards), with Galil Sniper (7.62x51 cal), but at 557 m (610 yards) it was 35-45 cm.
Added: heh mr White, same time replying basicly same thing