Anyone who steals things. As an example, I once stole some light bulbs out of the steam tunnels at my university, thus I am considered a thief.
Also a character archetype in role playing games, books, movies, and stories. They can be good, bad, or somewhere in the shades of grey - but always in the shadows.
"We own the night. Everything else, we have to steal."
Also a kickass computer game in which you go about hiding in shadows and sniping people with a bow; you can also move silently, knock people out, and other such fun stuff.