Stem angle is one of the factors that determines handlebar height. A listed angle of 0 degrees actually provides a rise of 16-20 degrees, depending on the bike's head-tube angle. Flexible, athletic riders prefer lower-rise stems, while recreational riders usually favor higher-rise stems.
Stem length is measured along the side of the stem from the center of the steerer to the center of the handlebar clamp (see diagram). Keep in mind that as a stem provides more rise, its effective length decreases. In the example above, if the +17-degree stem measures 135 mm, it actually extends 108 mm parallel to the ground.