Visa versus Visa Status
A nonimmigrant visa is a stamp/sticker that the US Embassy or consulate places on a page of someone’s passport, indicating the kind of nonimmigrant status that s/he is seeking when getting to the US. This visa stamp/sticker gives the person the right to request entry into the US in that type of status. However, the visa stamp alone does not give a person the right to enter the US, as other documents must be shown with it, and the final decision to allow entry into the US rests with the Customs and Border Protection officer.
Further, the only time that a non-immigrant is required to have a valid visa is when that person enters the US from abroad. The validity of the visa does not affect how long a non-immigrant may stay in the US.