If you are returning to the U.S. by air or sea, you will be asked to complete a Customs Declaration form. To "declare" means to tell the Customs officer about anything you're bringing back that you did not have when you left the United States. In addition to declaring items, you will be asked to tell what you paid for each item (in U.S. currency, including any taxes) and may be required to pay a “duty” (tax on imported goods) on them.
The following is a list of all items that must be declared on this form:
Keep in mind that if you leave the country with a foreign item, and do not have proof that you left with it, you may be required to pay a duty on it when you return. Sales receipts, appraisals and insurance policies serve as proof of prior ownership, or you may wish to register the piece before you leave. Registration needs to be done in person, before you leave the country, at a Customs office. Once you have received a Certificate of Registration (Form 4457) for a particular piece, it can be used over and over again in the future; you do not need to re-register the item each time you travel.
This is only a guide. For more information, please visit the U.S. Customs and Border Protection website.