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.

I plan to visit Ha Llong next summer holiday!
Wait to see the beauty of Ha Long bay….
I was in Halong Bay in 2007. The view in indeed breathtaking. It's not the cliffs themselves that make Ha Long Bay unique, but rather their sheer number. A huge bay, dotted with nearly 2,000 mostly uninhabited limestone cliffs, the breathtaking scenery is very similar to that of the Andaman coast of Thailand, Vang Vieng in Laos and Guilin in China.
I stayed on board Bhaya Cruises
I was there in October and woke up one morning because a snack boat lady was tapping my bedroom window with her oar. Pretty incredible.
I've been to Vietnam 10 years ago. I think the boat in the photo are the ones that sells various goods including souvenir items.