Excerpts from US Citizenship and Immigration Services. For more information, click on http://www.uscis.gov/portal/site/uscis

A nonimmigrant is a foreign national seeking to enter the United States (U.S.) temporarily for a specific purpose. Nonimmigrants enter the U.S. for a temporary period of time, and once in the U.S. are restricted to the activity or reason for which their visa was issued. They may have more than one type of nonimmigrant visa but are admitted in only one status. 

Fiancé(e) Visa

U.S. citizens who will be getting married to a foreign national in the United States may petition for a fiancé(e) classification (K-1) for their fiancé(e). You and your fiancé(e) must be free to marry. This means that both of you are unmarried, or that any previous marriages have ended through divorce, annulment or death. You must also have met with your fiancé(e) in person within the last two years before filing for the fiancé(e) visa. This requirement can be waived only if meeting your fiancé(e) in person would violate long-established customs, or if meeting your fiancé(e) would create extreme hardship for you. You and your fiancé(e) must marry within 90 days of your fiancé(e) entering the United States.  Until the marriage takes place, your fiancé(e) is considered a nonimmigrant.

You may also apply to bring your fiancé(e)'s unmarried children, who are under age 21, to the United States.  Please note, your fiancé(e) will initially receive conditional permanent residence status for two years. Conditional permanent residency is granted when the marriage creating the relationship is less than two years old at the time of adjustment to permanent residence status.

Student Visas

The Immigration and Nationality Act provides two nonimmigrant visa categories for persons wishing to study in the United States. The "F" visa is reserved for nonimmigrants wishing to pursue academic studies and/or language training programs, and the "M" visa is reserved for nonimmigrants wishing to pursue nonacademic or vocational studies.