MIL vs MOA are just two different units of measure when dealing with optics. MILs are based in centimeters and MOA is based in inches. There is a ton more information available as this is the simplest way to think about the difference between MIL vs MOA, so I would recommend doing more reading on the differences.
It's not about centimeters and inches. MIL is based on radians (milliradians). One radian is 57.30 degrees. There are 2xPi radians in a circle (6.283). There are 360 degrees in a circle. A minute of angle (MoA) is 1/60 of a degree. There are 21600 minutes of angle in a circle and there are 6283.18 MILs in a circle, so there are 3.44 MoA in a MIL. Practically, these small angle slices come out to about 1" difference at 100 yards for an MoA and 3 1/2" at 100 yards for MIL.
The difference between MIL and MOA is NOT how Armando describes, both MIL and MOA can be measured in cm or in. It can be rather involved to describe here, I would suggest doing further research on your own. There are many articles that describe each and their differences, preference on which is better is mostly based on the end user.
The difference between the MIL & MOA models is the adjustment increments being in MOA or MIL and the reticle used the MIL has a graduated chrismas tree style reticle whereas the MOA has hold overs for windage and elevation for 62 grain 5.56 out of a 16in. Barrel out to 1000 yards in 100 yard increments and 15mph in either direction in 5 mph increments if i remember correctly.
MILs are roughly 3.47 MOA