A looter is someone who takes advantage of a social crisis in order to steal some stuff. Many will claim that they are acting out of some sense of retribution relating to whatever incident sparked the crisis to begin with, but that's a bunch of bullcrap. Looters just want to take your stuff.
Actor Bryan Lugo played a looter in the "Human for a Day" episode of Supergirl. During an earthquake in National City, the looter held up a convenience store. A powerless Supergirl entered the building and appealed to the man's humanity, convincing him to surrender his weapon to her.