This article is from the FAQ, by with numerous contributions by others.
The K class visa is the "alien fiance(e)" visa, which an nonimmigrant
visa. A US citizen who wishes to marry a foreigner may file a K
class petition (I129F) which, when granted, allows the foreigner to
enter the US and marry within 90 days of arrival. Once married, the
foreigner should file for conditional permanent residence. There are
two K class visas:
K1 - for direct fiance(e)
K2 - for children of fiance(e)
 
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