What it means to be 'lightly wounded'

Whenever there is - God forbid - a terror attack, the wounded are generally classified as 'light,' 'moderate' and 'serious' with some even being classified 'critical.' Sometimes wounded are classified as between two of the categories.But what do these categories mean? Gila at My Shrapnel doesn't tell us what her initial classification was, but I am going to guess that it was either 'light' or