In transit confused??

My enquiry is in regards to a trip planed to the USA via Shanghai China. We will have a transit stay of 18 hrs in Shanghai, and would like to take the opportunity to sightsee the city while we are there. My question is, do we require a visa, or are our travel documents sufficient for our short stay in China. I have noted various comments posting 24 hrs or 48 hrs transit stay dose not require a visa, others say you can not leave the airport without a visa ?? Any feedback would be greatly appreciated.