K-1 Visa Overview

If you are an American citizen and are planning on marrying a foreigner you have the option of bringing you fiancé to the United States on a temporary non-immigrant visa to marry and live here. You may also apply to bring your fiancé's unmarried children, who are under the age of 21, to the United States via the same visa petition.
Legal permanent residents (green card holders) may not file petitions for the fiancé(e) visas, although they may petition for the immigration of their new spouse after the wedding.
The K-1 visa is a temporary visa to the United States for the sole purpose of getting married within the United States. This visa is not a permission to stay in the US for long term. You have 90 days to get married after your fiancé arrives in the country. If he/she plans to stay/work in the US, you will have to file another petition and application in order to adjust his/her status to a permanent resident (green card holder).