Types of visas

A Visas (Official Visas)-
A-1: For Ambassadors, public ministers & consular officers
A-2: For immediate family members of A-1
A-3: Attendants & servants of A-1 and A-2 holders

B (Business/Visitor) Visa -
B-1 Temporary visitor for business
B-2 Temporary visitor for leisure

C & D Visa (For Aliens in transit) -
C-1,2: Alien in transit directly through US
C-3: Family of C-1,C-2 in transit
C-4: Transit without Visa(TWOV)
D-1: Sailors departing on vessel of arrival
D-2: Sailors departing by other means

E - Visa (For Traders/Investors) -
E-1 Treaty Trader, spouse and children  
E-2 Treaty Investor, spouse and children  

F Visa (Students) -
Want to study or research at a U.S. college? Then F is the visa for you -
F-1: Academic Student
F-2: Spouse or child of F-1

H (Temporary Worker) Visa -
H-1B: Persons in a specialty occupation
H-2B: Seasonal nonagricultural workers
H-3: Trainees other than medical/academic; also training of handicaps
H-4: Dependants of H visa holders

I Visa (Mediapersons) -
Are you a reporter, film person, Editor? Then you require an I-visa -
Essential docs: Your press ID, a letter from the editor.
 
J & Q Exchange Visitor Visa -
J-1 exchange visitors may be academics, scientists, businesspeople or students.
J-1: Visas for exchange visitors
J-2: Spouse or 'child' of J-1 under 21

Fiance(e) of US Citizen -
K-1 Fiance(e)
K-2 Minor child of K-1
K-3 Spouse of a U.S. Citizen (LIFE Act)
K-4 Child of K-3 (LIFE Act)
Docs: Marriage certificate & Photos, Intent of marrying within 90 days in US(for K1).

L Visa (Intracompany Transferees)-
L-1A Executive, managerial
L-1B Specialized knowledge
L-2 Spouse or child of L-1

Vocational and Language Students -
M-1 Vocational student or other non-academic student
M-2 Spouse or child of M-1

O -Visa (For Prodigies) -
O-1: For a Genius in Sciences, Arts, Education, Business, or Athletics.
O-2: Alien's (support) accompanying O-1
O-3: Spouse or child of O-1 or O-2

Athletes and Entertainers-
P-1: Athletes & Entertainment groups
P-2: Entertainers in exchange programs
P-3: Entertainers in cultural programs
P-4: Spouse or child of P-1, 2, or 3

R-Visa (Religious Workers)
R-1 Religious workers
R-2 Spouse or child of R-1