Immigration can be confusing for multiple reasons, and one of these reasons relates to how many different types of visas there are. One of the more popular varieties of visa is the K-1 visa. 
However, a very limited group of people are eligible for a K-1 visa. According to the US State Department, a K-1 non-immigrant visa is specifically for individuals who are coming into the United States with the intent of marrying a US citizen. 
What are the requirements of a fiance? 
You might believe that the only requirement between fiances is intent to marry, but the reality is a little bit more complicated than that. To qualify as a fiance, both parties need to be legally free to marry when the parties petition for a K-1 visa. This means that if either party divorced in the past, there needs to be legal proof of the divorce going through. 
Additionally, generally both the US citizen and the non-US citizen have to have met in person within the previous two years to demonstrate eligibility for the K-1 visa. However, there are sometimes exceptions offered to this requirement, particularly if the reason for not meeting is due to hardship. It is also possible for the US government to grant a waiver in the event that there are cultural prohibitions on meeting prior to marriage. 
What else should I know? 
In the event that the immigrant spouse has eligible children, those children may come into the United States with a K-2 visa. It is also important to know that same-sex couples have the same benefits as opposite-sex couples under current US law.