BEFORE "-":
the letters indicate the engine family, for example in 18R-G
the engine family is R, in 2JZ-GTE the family is JZ.
The number(s) at the beginning is the number of the are for a certain
bottom end version (block, bore, stroke). The larger the number,
the newer the bottom end version. It may seem like the larger the
number, the larger the displacement but this is not always true.

AFTER "-":
G = twin cam (wide angle, 45 degrees or more between the intake and exhaust valves)
F = "economical" twin cam (narrow angle, around 22 degrees)
T = turbocharged
Z = supercharged
E = fuel injection
i = single point fuel injection
L = transverse mounted engine (seems to be an obsolete code)
B = twin carbs (only used on non-twin cam engines, obsolete code)
R = air injection
S = swirl intake ports (only a few made in mid '80s)
S = direct injection & swirl pot pistons (starting from '97/98)
U = emission package (Japan)
C = emission package (California)
LPG = LPG fuel