What is referred to as a U.S. visa stamp is actually a sticker, as in the example to the left. The visa is issued in the visitors passport by a U.S. embassy or consulate outside the United States. The visa indicates that the visitor is eligible for entry and reentry to the United States in a specific immigration category (F-1, J-1, H-1B, O-1, etc. ..) The validity length of the visa varies greatly based on geopolitical agreements between the U.S. and other countries. To look up how long a specific visa might be issued for please review the U.S. State Department Reciprocity by Country site. Most visas allow for multiple entries.
Immigration status is your legal permission to remain in the U.S. under specific conditions as defined by a visa document. For example an F-1 student would have a Form I-20 as a visa document. A J-1 scholar would have a Form DS-2019 and an H-1b professor would have an Approval Notice (form I-797.) In most cases the visa document will be (or will be extended to be) for a longer period than the visa stamp discussed above. For most international travel both the visa document and the visa stamp must be valid.
Visa Application Instructions
U.S. Department of State