I've made mention of light, medium, and heavy armor before in particular to weapons' characteristics and even way back when I discussed skills. Let's put a formal definition in place.
Quick descriptions: leather represents any armor made from hides, cloth, or other types of padding. Scale is metal components sewn or backed onto padding material. A chain hauberk is the typical representation of mail. Plate mail utilizes a series of plates with mail used to cover the joints between them. Full plate characterizes a high quality steel (not iron) coat of plates with mail backing.
EDIT: The way the chart above looks, it appears that a shield imparts a total AC of -1. In actuality, it just reduces AC by 1 point. Everyone familiar with D&D should understand that, but I wanted to make it clear.