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.