The helmsman (or helm officer) was the Starfleet crewperson who operated the helm console on 23rd century Federation starships.
The helmsman of those vessels worked in concert with the navigator, who plotted the ship's course. On the USS Enterprise (NCC-1701)the helmsman controlled both the speed and attitude of the ship, as well as the ship's weapon and shield systems. Other spacefaring services use the term helmsman also, the 22nd century Earth Starfleet for one, but it isn't clear if there is the same relationship with a navigation console, since vessels like Enterprise (NX-01) had a single helm station.
By the 24th century, the helm and navigation stations became combined as the flight controller position. Also, control of weapons and defenses is usually handled by the tactical station or the operations manager, rather than their 23rd century equivalents, under the control of the helmsman.
One possible explanation as to why the helm and navigation stations were separate in the 23rd century but not before is that the combination of flight control, weapons control, and navigation would have been too much for one officer to handle. For this reason, navigation would be assigned to a separate officer, and the stations became combined again later after the decision was made to once again make weapons control a seperate entity.