Form I-129F is the form that a U.S. citizen files for his or her foreign national fiancé or fiancée. The fiancé(e) visa is in the K-1 visa category. K-2 visas are available for the minor dependent children of the foreign national fiancé or fiancée.
The U.S. citizen files form I-129F with supporting documents in the United States. If USCIS approves the I-129F petition then the foreign national fiancé or fiancée will eventually apply for a K-1 visa at the designated U.S. Consulate abroad. If the foreign national is approved for a K-1 visa and is admitted to the U.S., the U.S. citizen and foreign national must get married within 90 days.
Then the foreign national must apply for a green card using the Adjustment of Status process. Same-sex couples are eligible in this category. That can be useful if it is difficult or impossible to get married abroad because of laws preventing same-sex marriage in particular countries. The marriage can occur in the United States where it is legal.