Visas are required by most foreigners entering mainland China although, at this stage, visas are not required by Western nationals visiting Hong Kong and Macau. Visas are available from Chinese embassies and consulates in most countries.
Customs Overview
Don't bring any cold meats or fresh fruit with you - Chinese law forbids their import. There are limits to other items, such as herbal medicine, that you can take out of the country. Rare animals and plants also cannot be exported.
Cultural relics, handicrafts, gold and silver ornaments, and jewellery purchased in China have to be shown to customs on leaving (but checks are fairly cursory these days). If these items are deemed to be "cultural treasures", they will be confiscated. All bags are X-rayed.
It's illegal to import any printed material, film, or tapes "detrimental to China's politics, economy, culture and ethics"; this includes pornography. But don't be too concerned about what you take to read.
As you leave China, any tapes, books etc "which contain state secrets or are otherwise prohibited for export" can be seized.
Duty Free
You're allowed to import 400 cigarettes (600 if you are staying more than six months), four bottles of wine or spirits and a reasonable amount of perfume. Cash amounts exceeding 5000.00 (or its equivalent in another currency) should be declared.