The current in Hong Kong is 220 volts, 50 cycles alternating current (AC), so most American appliances can't be used without a transformer. Exceptions are most laptops and mobile phone chargers, which are dual voltage (i.e., they operate equally well on 110 and 220 volts), and thus require only an adapter. The same may be true of some hair dryers and other small appliances. Always check labels and manufacturer instructions to be sure. Don't use 110-volt outlets marked "for shavers only" for high-wattage appliances such as hair dryers.
Most plugs have three square prongs, like British plugs, but you can buy adapters in just about every supermarket and at electronics stalls in street markets. If you travel frequently, consider making a small investment in a universal adapter, which has several types of plugs in one lightweight, compact unit.
Walkabout Travel Gear has a good coverage of electricity under "adapters."
Walkabout Travel Gear (800/852–7085 in U.S. www.walkabouttravelgear.com.)