A single letter by itself refers to a clockwise face rotation in 90 degrees (quarter turn):

A letter followed by an apostrophe means to turn that face counterclockwise 90 degrees:

A letter with the number 2 after it marks a double turn (180 degrees):

U1
U2
U3
U4
U5
U6
U7
U8
U9
F1
F2
F3
F4
F5
F6
F7
F8
F9
D1
D2
D3
D4
D5
D6
D7
D8
D9
B1
B2
B3
B4
B5
B6
B7
B8
B9
L1
L2
L3
L4
L5
L6
L7
L8
L9
R1
R2
R3
R4
R5
R6
R7
R8
R9