Now that most offensive formations have only one or two running backs, the original designations do not mean as much, as the fullback is now usually a lead blocker (technically a halfback), while the halfback or tailback (called such because he stands at the "tail" of the I) lines up behind the fullback.

Why is it called a tailback?

Tailback can mean: A position in American football at the "tail" end of the offensive formation, typically a halfback. A line of motor vehicles caught up in traffic congestion; a traffic jam.

Are halfbacks and running backs the same?

A running back may be a halfback (in certain contexts also referred to as a "tailback" ⁠ ⁠— see below), a wingback or a fullback. A running back will sometimes be called a "feature back" if he is the team's starting running back.