A bachelor party is held specifically for the groom to celebrate his upcoming nuptial. The best man is usually the host of the event. The cost is paid for by the best man and/or his groomsmen and is for select male guests only.
A stag party is an open event. Tickets for the event are sold to raise funds to help pay for the wedding. It is also a way to include acquaintances and associates who are not close enough to the couple to be invited to the actual wedding or reception. This event generally includes party games, prizes, dancing and food.