To come to China you must have valid travel document (passport) and visa. Visas are issued by consular representations of PRC abroad.
One can get visa on arrival at certain points of entry if invited by authorized organization and had no sufficient time to apply for visa prior to arrival. If so, must hold proof thereof and be met by sponsor.
People's Republic of China signed bilateral treaties with some countries allowing certain categories of visitors visa-free entry.
Here is the list of visitors enjoying visa exemption:
Mongolia (with supplementory yellow list), Romania, Russian Federation (provided travelling within touristic group on invitation of authorized Chinese tourist agency), San Marino
Normal passports endorsed "Service" or "For Public Affairs"
Armenia, Azerbaijan, Benin, Bosnia Herzegovina, Bulgaria, Cuba, FYROM (Former Yugoslavian Republic of Macedonia), Georgia, Kyrgyzstan, Moldova, Pakistan, Tajikistan, Turkmenistan, Ukraine, Vietnam, Yugoslavia
Official or service passports
Albania, Argentina, Azerbaijan, Bangladesh, Belarus, Benin, Bolivia, Chile, Colombia, Croatia, Cuba, Cyprus, Czech Republic, Ecuador, Georgia, Hungary, Iran, Jamaica, Jordan, Kazakhstan, Korea (North), Laos, Latvia, Lithuania, Maldives, Mexico, Mongolia, Myanmar, Pakistan, Peru, Poland, San Marino, Seychelles, Slovak Republic, Slovenia, Sudan, Thailand, Turkey, Venezuela
By the way, Hong Kong and Macau have immigration rules of their own. Visas for these special administrative regions issued separately. For regulations of Hong Kong, visit the site of HK Immigration Department.