The visa stamp that is placed in a your passport merely evidences that you have been to the U.S. Consulate or Embassy and have been determined by a consular officer to be eligible for the specific visa and may now seek entry into the U.S. Having such a visa DOES NOT guarantee that the Department of Homeland Security will allow you to enter once you present yourself at the Port of Entry with the visa. Further, the dates provided on the visa merely show how long you have to present yourself at the Port of Entry into the U.S. to request admission. Note that the visa may allow multiple entries in certain cases and may be valid for up to 10 years as is sometimes seen with visitor visas.
Once you present yourself for inspection at a U.S. Port of Entry using your visa, and assuming you are granted entry, you will receive an I-94 card in your passport which will indicate the visa status you have been allowed entry on as well as how long you are being admitted for. It is this date that determines the length of stay you have been granted and NOT the date on your visa stamp.
If you have a 10 year visitor visa stamped in your passport and you present yourself for inspection at the Port of Entry of your choice, the office, assuming you are granted admission, will then provide you an I-94, usually valid for 6 months. You are then allowed to remain in the U.S. for those 6 months as indicated on your I-94 and not for 10 years as indicated on your visa stamp.