Roof pitch is one of the most used terms in roofing.
So what does the 7/12 in the example to the right mean? 
The 7 means that the roof rises 7" for every 12" it runs. The example to right shows 2 different ways to determine or calculate roof pitch, also called roof slope.

  In Example A, the measurements are taken from on top of the roof. Simply mark a level at 12", hold it perfectly level and measure from the roof surface to your 12" mark, this will give you the rise. Be careful with this method as a roof with many layers of shingles, or any type of roofing that is irregular can give you less than precise results.

  In Example B, the measurements are taken from the bottom of the rafters, the procedure to calculate  roof pitch is the same. This method tends to be more accurate because no layers of roofing are involved. To use this method there are 3 possible places to take your measurements. 1. The underside of a barge rafter on a gable end. 2. The underside of a rafter on a overhang at the bottom of the roof  3. The underside of a rafter in the attic. 

 Terminology used to describe roof pitch or slope, include 7/12, 7-12, 7 to 12,7 and 12, 7 on 12.

To see roof pitch to degrees equivalents go here
To see roof pitch visual examples go here.