So for the average golfer we will briefly sum up what the numbers mean and what affects them.

Here it is:

You will typically see a course rating on score card in this format:
Course Rating: 71.5/116

Now the first number in the course rating is actually the course rating .
That number is calculated by having a scratch golfer (a golfer who normally golfs even after 18 holes) play the course under regular conditions and take the best half of his scores for the course and average them out per hole. Then that number is multiplied by 18 to get the course rating. So an easier course can be a par 72 but have a course rating of 69.1 if it is really easy.

The second number of the Course Rating is the course’s slope rating. This is the number that really gives some insight into the relative difficulty of the course. That scale works like this: The lowest slope rating is 55, the highest is 155, and a golf course of standard difficulty has a rating of 113. As the slope rating increases so does the difficulty. The slope rating is calculated as follows:
USGA Slope Rating =(Bogey Course Rating-USGA Course Rating) x 5.381 for men or 4.24 for women.
To get all the ratings needed for this calculation the course gets a yardage rating and an obstacle stroke value. Did I mention this can get kind of complicated. The slope rating takes into account the effective playing length which is the measured yardage plus adjustments made for roll, forced lay-ups, elevation, wind and altitude.
Then you get to the obstacle stroke value which is a evaluation of all the course obstacles( Out of bounds, water, trees, bunkers, recovery and rough, topography, fairway, green target, green surface, and psychology).

You see what I mean. There is a lot to take in. So for basic reference, low slope easier, high slope more difficult, average of about 113. If the course rating is less than par for the course it plays a little easier, but the slope rating will give a golfer his best basis for the relative dificulty of a course. For a more in-depth breakdown of the whole course rating system check out the USGA Website. It includes definitions for many of the terms mentioned above and more in-depth information.