In American football, there are various position designations for runners that carry the ball across the line of scrimmage: full, half and running back.
Are there any differences between the three positions in terms of roles?
Running backs come in two types: fullbacks and halfbacks (aka tailbacks).
By default, if somebody is talking about a running back without specifying, they're probably talking about the halfback/tailback, because this is the position that carries the ball most of the time and has the most visibility and name recognition.
The fullback is used as a lead blocker, running out in front of the play to set blocks and clear a path for the tailback.
Initally, the quarter, half, and fullbacks were deployed a quarter, half, and full yard behind the line of scrimmage.
The quarterback has the most versatile role of the three "back" types, because he can either pass the ball, or run, if the defense has kept its men back to cover his "receivers."
The halfbacks were supposed to use speed to run to either end of the line, either past the end (end run) or behind the tackle (off tackle).
The fullback was supposed to charge through the middle of the opposing line by using power. This could be done successfully only if previous passing or end-running plays caused the defense to weaken the center of the line to reinforce their flanks. Alternatively, an unsuccessful "draw play" by a fullback might "set up" a flanking running or passing play. Otherwise, a fullback may be a blocker for a quarterback who wants to pass, or a halfback who wants to run to the side.