Hello, if you stay in the terminal building then you don't need a visa, but you need a visa if you want to go out of the building.

Are you sure you need a visa? The information on this very page says that you can leave the airport if you're a US citizen staying less than 48 hours.

After four trips to China's embassy in Hanoi and $140 USD later, I'm finally in possession of the right visa. A few hard lessons I've learned in the process...
1. Transit visa can only be issued upon arrival in Shanghai and Beijing. If routed through another China airport (in my case, Guangzhou) en route to Beijing/Shanghai, you cannot get a transit visa at any other airport beside Beijing and Shanghai. You must have a tourist visa.
2. Transit visa are only for 24 hrs. The 48 hrs period has recently been shortened.
3. Americans are charged $140 for tourist visa, higher than most nationalities. Some nationalities can apply for transit visas in advance (which are about $30 for most nationalities) …but not Americans. Americas cannot get transit visas in advance.
4. Only foreigners who work in Hanoi and have proper paperwork can get a tourist visa from China's Hanoi embassy. Tourists are direct to their HCMC consulate.
As a word of caution: China's Hanoi embassy is chaos and scams artists linger out front. Avoid if possible. I witnessed three major arguments inside with foreigners fighting to get visas or retrieve their passports. Overall it's a very hostile and unpleasant environment.

Thx to Mr Niki and Mr Chan for your responses.