A temporary, non-immigrant visa permits its holder to apply for entry to the U.S. for a temporary period of time, for a specific purpose. That purpose might be to work, go to school, attend a conference, etc., or to visit the country and friends.
What distinguishes a non-immigrant visa from an immigrant visa is that the non-immigrant visa only allows a person to enter temporarily, whereas an immigrant visa holder can enter and stay permanently. Immigrant visas are discussed in the section called Permanent Visas.
At the border, the non-immigrant visa holder can be admitted to the U.S. in the particular immigration status indicated on the visa. How long someone can be admitted to the U.S. depends on which immigration status they are admitted. A person admitted in one status can often change their status in order to stay for a different purpose. For example, someone admitted to attend school may want to change their immigration status to one of the employer-sponsored non-immigrant visas once they graduate and find a job (provided that their new employer is willing to sponsor them). Several types of non-immigrant visas also allow a person to extend their status and thereby extend their stay in the U.S.