Countries charge import and export duties (also known as tariffs; both are essentially a tax on international trade) in order to protect domestic producers and raise state revenue. The U.S. has passed a number of free trade agreements and special status agreements in recent years which have lowered average tariffs, but the U.S. still charges duties on many products.
Electronics imported for personal use face the same duties as those imported for commercial use, with some exceptions. Travelers bringing back limited quantities (depending on the country of origin, valued at no more than $800 total) of electronics do not have to pay duties for the items and people moving to the U.S. do not have to pay duties on electronics that are part of their household items. Customs also does not charge duties on personal use shipments valued at less than $200 ($100 for gifts).
Country of Origin
When bringing electronics into the U.S., the country of origin makes all of the difference. The U.S. uses the Harmonized Tariff Schedule, which is similar to what most countries use and is based on international trade agreements and standards. There are three classifications when assessing duties: countries that the U.S. has normalized trade relations with (charged general duties), countries that the U.S. has a special relation with (through free trade or other agreements, charged no duty or reduced duties) and countries that the U.S. does not have a normalized relationship with (such as Cuba or North Korea).
The following rates are from the General Duty column of the 2010 Harmonized Tariff Schedule. Cell phones and accessories are free, and most consumer sound equipment is also free. Consumer cameras and video recording equipment are free, though some professional quality equipment and accessories may be assessed a 2% duty. Keyboards may be assessed a 5.4% duty while electric instruments carry a 5% duty.