One of the best resources on the web for all the sight units is found at What's MOA. Mils and other units are included down the page along with videos from the National Shooting Sports Foundation.

Here's an excerpt:

Quote
In practical terms the radius is the distance from the gun to the target and a change of 1 milliradian (MRad) changes the impact point of a bullet by 1/1,000 the distance to the target. If the target is 1,000 feet away a change of 1 MRad changes the impact point of a bullet by 1 foot. If the target is 1,000 yards away a change of 1 MRad changes the impact point of a bullet by 1 yard. If the target is 1,000 meters away a change of 1 MRad changes the impact point of a bullet by 1 meter. If the target is 500 meters away a change of 1 MRad changes the impact point of a bullet by 1/2 meter. The exact 1 to 1,000 ratio between impact change and range to the target in whatever units you use for range makes doing the math in your head relatively simple.