Stem angle is one of the factors that determines handlebar height. Flexible, athletic riders prefer lower stems and bars, while recreational riders usually favor higher stems/bars.
Stem length is measured along the top from the center of the steerer to the center of the handlebars (diagram).
Tip: Keep in mind that as stems provide more rise, their effective length decreases. In the example above, if the +17 Degree stem measures 135mm, it actually extends only 108mm when length is measured on a line parallel with the ground.