K-1 visas are for fiancees of US citizens allowing them to come to the US to get married. They are available for male-female and for same-sex marriages.
The first step to establish eligibilty for a K-1 visa is for the US citizen to submit a I-129F petition to the USCIS. Once the petition is approved, it will be forwarded to a US Consulate abroad where the fiancee will apply for his or her K-1 visa. The petition is valid for 4 months although this period may be extended by a consular officer.