The currency of China is the renminbi (RMB) or yuan (or colloquially known as 'kwai') though you will probably see Hong Kong dollars floating around southern China.  ATMs are abundant in most areas of China, and credit cards are widely accepted in metropolitan areas like Hong Kong, Shanghai, and Beijing.

While ATMs have become increasingly common and are generally very convenient, traveller's checks remain a viable option in China.  They can be exchanged at airports, banks, some large department stores, and many hotels at a rate slightly better than cash.  Exchange rates are regulated, so rates are generally about the same anywhere.

When exchanging money in China, it is important to retain exchange receipts.  These are needed to convert Renminbi back into foreign currency when leaving China.  You may convert up to half of the amount of Renminbi shown on your receipts. Alternatively you can take currency out of China and convert abroad though exchange rates will mean losses! Note that this only relate to cash exchange receipts. Withdrawal receipts from ATM's will not allow you to convert your RMB's back to foreign currency in China.

Credit cards are becoming increasingly popular and and are accepted in most hotels, shops and restaurants as well as in many supermarkets. Cash is king in China though, and cards are generally not accepted for causal expenses. Carrying a reasonable amount of cash to pay for drinks, snacks, buses, taxis and small souvenirs or other small purchases is therefore a sensible precaution.