Beam diameter and beam divergences are two of the most important technical specifications to consider when choosing a laser.

Obviously beam diameter is just the diameter of a laser beam, measured in millimeters. Beam divergence measures how much the laser beam expands per meter. For example, a laser with a beam divergence of 1.0mRad will have a beam that expands 1.0mm per meter.

How do these two specs relate to each other? and more importantly, why does it matter?

It all comes down to how small the end "spot"(the diameter size of your leaser beam) is at certain distances. This is important because you want to point far, far away and because you want to burn stuff, lots of stuff.

Common senses tell us that if a laser beam diameter is small, it can go further and burn with more power than if it is big. Ideally, we want to have small beam diameter at aperture AND small beam divergence.