From Wikipedia:
In a North American/Australian wedding a groomsman or usher is one of the (usually) male attendants to the bridegroom in a wedding ceremony. Usually the groom selects his closest friends and/or relatives to serve as a groomsmen, and it is considered an honor to be selected. From his groomsmen, the groom chooses one to serve as best man. In a typical British wedding ceremony, although there may be several bridesmaids there will be only one male attendant, the best man.