China requires a visa for travel. There is a business visa and a family visit/tourist visa. You can get a multiple entry visa if you have a letter saying you have family there or you have a letter from a business there. Get the forms from Chinese Visa, Passport, Notarization & Authentication.
"Additional Visa info":http://www.chinaonlinetravel.com/visa.htm. You do need a passport photo.
bq. The Chinese consulate requires a photo similar to the one in your passport. They will only accept photos which are head-only, full frontal, with a white background, approximately 1-1/2" by 2". You can obtain this type of photo anywhere that advertises "passport photo" service. Cutouts from personal photographs are not acceptable. There are bunch of services that take care of this.
By the way, US Citizens don't need visas to enter Hong Kong (technically the Hong Kong Special Administrative Region) if they are there for less than 90 days.