Domestic electricity supply voltage varies between countries. Over 30 countries (including those in North America) use a voltage of 110V~130V, while another 120 countries (including most of Europe), use a voltage of 220V~230V. The electricity in China is generally 220V, 50HZ, AC (Hong Kong is 200V; Taiwan is 110V), while the supply voltage of Japan is 100V, 50/60HZ.
If you travel to China and wish to bring electric devices for use during your stay, a transformer, which can be bought in China for CNY100-200, is necessary. Most of the hotels in China have both 110V and 220V electrical outlets in the bathrooms, though in guest rooms usually only 220V sockets are available.
As the shape of a socket varies between countries, a portable plug adaptor may also be necessary. It can be purchased from travel stores (such as Franzus) or electronics stores (i.e. RadioShack, Best Buy) in your local countries. Also you can buy it in China.
Cell Phone Charger
You may bring you own phone and laptop to China. Cell phone and laptops chargers almost all come with a built-in voltage transformer. You can check on the cord or appliance to verify this (it will say range 110v-240). Laptops it says on the charger unit on the cord. As for all other appliances, some have transformers but many do not, so check in the manual of the individual appliance.
Standards of Sockets in China
This is what most wall outlets like these days in China. The top takes a two-pin plug. The teeth should be of the same size ("Type A"), although many modern appliances with type A plugs have a wider pin. This type does not fit into the Chinese outlet and require an adapter. This socket will also be a "Type C" or "F Type" plug which is the norm in Germany. (If you come from Europe, all your devices work - China uses the same voltage.)
Taking down of the photo is "Type I" plugs common in New Zealand and Australia. (All devices your work if you are from Australia / New Zealand and you use the same voltage as China.)
Read More on Cell Phone Accessories for travel
Read More on How to Make Calls in China
Read More on How to Make Calls in Hong Kong