A "USGA Course Rating" is the United States Golf Association's mark that indicates the evaluation of the playing difficulty of a course for a scratch golfer under normal course and weather conditions. It is expressed as strokes taken to one decimal place, and is based on yardage and other obstacles to the extent that they affect the scoring ability of a scratch golfer.
A "Slope Rating" is the USGA's mark that indicates the measurement of the relative difficulty of a course for players who are not scratch golfers compared to the USGA Course Rating (e.g. , compared to the difficulty of a course for scratch golfers ). A Slope Rating is computed from the difference between the Bogey Rating and the USGA Course Rating. The lowest Slope Rating is 55 and the highest is 155. A golf course of standard playing difficulty has a Slope Rating of 113.
Source: USGA Handicap System Manual Section 13