Halfback

What he or she does
The halfback or the running back as they are also called sets up either next to the quarterback in the shotgun or in a normal formation behind the quarterback and/or fullback. But of course they could set up as a wide reciever or while in the wildcat formation could possibly be the QB.

Examples
Some examples of a running back or halfback would be:

Mark Ingram Alabama Crimson Tide

Jeffrey Demps Florida Gators