Northwest will board wheelchair passengers first, then offer pre-boarding for families travelling with small children, persons needing extra assistance, first class passengers and usually the elite members (NW Platinum, Gold, Silver elites, plus elite level flyers in the CO or DL programs), then if they have time, exit row passengers, then they do general boarding from the rear of the plane.
(I've seen elite preboarding skipped when they were running late, same with exit row preboarding)