List of some of the most common type US Immigrant Visa Categories (and types):
F: Students: The F-1 visa is required to enter the US as a student for pursuing a full-time study program.
H: Professionals of Special Occupations (H-1 to H-3): Professionals desirous of working temporarily in the US can avail of the H visa.
J: Exchange Visitors/ Student Programs: The J or J-1 visa is issued since 1961 to promote cultural exchanges between US and rest of the world.
K: Future Spouses: This visa enables an American citizen to bring his/her foreign born spouse to live in the US or marry his/her fiancé in the US.
L: International Companies: The L1 is a non-immigrant visa which allows companies operating both in the US and abroad to transfer certain classes of employee from its foreign operations to the USA operations for up to seven years.
M: Non-Academic and Vocational Students: The M visa is similar to F visa except that it is reserved for individuals pursuing vocational or nonacademic studies.
R: Religious Workers: Foreign religious workers who have been invited to work in the US by a religious organization can avail of the R-1 nonimmigrant visa.