Complex sentence: A fullback in American football is very different from a fullback in rugby league, some of the positions are fairly similar; the stand-off/five-eighth and halfback carry out a similar role to a quarterback in American football.
Split into simpler sentences: A fullback in American football is very different from a fullback in rugby league. However, some of the positions are fairly similar: for instance, the stand-off/five-eighth and halfback carry out a similar role to a quarterback in American football.