From the World Heritage inscription:
Ha Long Bay, in the Gulf of Tonkin, includes some 1,600 islands and islets, forming a spectacular seascape of limestone pillars. Because of their precipitous nature, most of the islands are uninhabited and unaffected by a human presence. The site’s outstanding scenic beauty is complemented by its great biological interest.
Ha Long Bay might be the best natural heritage site in SE Asia (excluding Borneo).
They typical Ha Long Bay excursion will be a 3-4 hour bus ride from Hanoi and a 1 or 2 night stay on a junk in Ha Long Bay. There are also options to stay in nearby Cat Ba National Park.
One of the unique features of Ha Long Bay are the women who sell snacks and drinks from their boats. They will row up to the anchored junks to sell things to tourists. The woman in the above photo had a boat full of soft drinks, potato chips and candy.