It is usually measured in units of thousandths of an inch. Generally, a group of strings will be named after the thickness of the first or thinnest string in the group. You may hear someone say they use "10". This just means that they are using a set of guitar strings, where the first string is 010 inches thick.
Anything lighter than 012 is considered a custom light or super light. Usually, the heaviest acoustic guitar string you will see will be the medium or 013 gauge.