Overview

Except in the case of a refugee or an individual granted political asylum in the United States, an immigrant visa must be approved before a foreign national can become a permanent resident of the United States.
There are two primary ways an immigrant visa can be granted: through the petition of a close family member, and through the petition of an employer. These petitions can be filed for people already in the United States, or people in other countries.

Not all approved immigrant visa petitions entitle the beneficiary (that is, the individual on whose behalf the petition was made) to apply to become a permanent resident immediately. There are a limited number of immigrant visas available each year for each category, depending on the relationship of the beneficiary to the petitioner or the type of job the beneficiary will perform for the petitioning employer. In some of the visa categories, there is a substantial backlog of previously approved visa petitions. An immigrant visa petition maintains its place in the backlog based on the date it was filed, or its "priority date."

Where are Visas issued?
Visas are issued at American consulates in other countries. They may be issued as the result of an application made to the Bureau of Citizenship and Immigration Services in the United States by a relative or employer; or as the result of an application made to the consulate itself by the individual seeking the visa. For many of the nonimmigrant visas, immediate family members will qualify for a "derivative" visa, based on their relationship to the primary visa holder. However, each family member must apply for and be granted the derivative visa on his/her own; he/she may not enter the United States based on the visa in the family member's passport. A nonimmigrant visa may be issued for a very brief period of a few weeks, to coincide with the applicant's travel plans, or for many years, depending on the type of visa.

Foreign Consulates in the U.S. and U.S. Consulates abroad
http://www.state.gov