Chain mail (also spelled chainmail, chain maille, or chainmaille) dates back to about 300 B.C. as a light weight armor that resists piercing and slashing. It was used in many cultures including the Celts, Romans, Europeans, Persians and Japanese. Chain mail is still in use today for things like butcher gloves and shark suits - as well as jewelry.
Chain mail is created by taking metal wire rings and linking them together in various patterns. These patterns are called weaves. Where stability is crucial - like in butcher gloves or shark suits - the rings are soldered or riveted closed to ensure they do not break open when force is exerted; however, jewelry rings are normally not closed this way. This greatly reduces the time to create the jewelry and also makes it easy to modify in the future.
The thickness of the wire used to create the ring is called the gauge - the same as with electrical wire. The higher the gauge the thinner the wire. Typical wire gauges are 14g, 16g, 18g, and 20g - though lower and higher gauge wire is available.
The ring size is the inner diameter - the diameter of the hole of the ring - and is usually roughly measured in fractions of an inch - 3/8", 1/2", 7/16", etc - though the actual measurement can vary. One 1/2" ring may be 0.561" while another may be 0.483".
The ratio of the ring's wire gauge to its inner diameter is called the ring's aspect ratio (AR). AR is very important in chain mail because it determines how loose or tight the weave is - each weave has its own optimal AR with a range that can effectively be used.