This article is from the Golf FAQ, by marcelo@nntpserver.Princeton.EDU (Marcelo A. Gallardo) with numerous contributions by others.
The following information is a brief explanation. For more detailed
information, please review the USGA Handicap Manual available from the
"Yardage Rating" is the evaluation of the playing difficulty based on
yardage only. It is the score a scratch player on his game is expected
to make when playing a course of average difficulty.
USGA COURSE Rating
"Course Rating" is the evaluation of the playing difficulty of a course
for scratch players. Course Rating is expressed in strokes and decimal
fractions of a stroke, and is based on yardage and other obstacles to
the extent that they affect the scoring ability of a scratch player.
Courses are rated by authorized golf associations, not by individual
USGA SLOPE Rating
"Slope Rating" reflects the relative playing difficulty of a course for
players with handicaps above scratch, whereas Course Rating is based
solely on difficulty for the scratch player. The lowest Slope Rating is
55 and the highest is 155. The average Slope Rating for men and women