Voltage varies between countries. Over 30 countries (including those in North America) use a voltage of 110V~130V, while 120 other countries (including most of Europe), use a voltage of 220V~230V. The electricity in China is generally 220V, 50HZ, AC (Hong Kong is 220V, Taiwan is 110V).
If you visit China and wish to bring 110V-only 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 220V sockets are usually the only type available.
As the shape of a socket varies between countries, a portable plug adapter may also be necessary. It can be purchased from electronics stores (i.e. RadioShack, Best Buy) in your local countries. They may also be purchased in China.