Starboard is the right side of a vessel as one faces forward, the left side being the larboard, or port side. The name derives from Old English steorbord. In the early days of sailing, before the rudder was placed on the centerline at the stern, a ship was steered by a 'steering board', usually on the right side of the ship, hence the term.
Since the steering oar was on the starboard side, a vessel would dock on the left or larboard side. The term larboard was used to denote the left side of the vessel. The name was derived from the middle English ladebord, lade being a term for 'load'.
Since the terms larboard and starboard sound similar and could be confused, later larboard was replaced by port, since that was the side docked in port. To eliminate ambiguity, since the term left and right is relative to the observer, the terms larboard and starboard, or later, port and starboard, is used relative to the ship.