Although it usually doesn't make a lot of difference what something is called, I have been campaigning for years on this forum to avoid calling the servo pulses PWM because analogWrite is the way Arduino produces PWM and driving a servo using Arduino analogWrite can destroy servos.
PWM involves the modulation of a signals duty cycle. In controlling a servo, the duty cycle does not control the position of a servo. Servo position is controlled through modulating the on time of the pulse. Indeed it is possible to control a servo over its full range of movement by changing the on time but keeping the duty cycle constant. If servo control works perfectly well without changing the duty cycle then PWM modulation would not be the most appropriate term for the modulation used.
The terminology is confusing because servo control pulses do involve changing (modulating) the pulse width. And a subset of PWM can be used to emulate the modulation needed to control a servo. But its no more correct to say that this makes the servo signal PWM than it would be to say that an AC (Alternating Current) signal is PWM because PWM is capable of alternating (switching on and off) current. Indeed it is possible to emulate AC using PWM (along with a few passive components) but of course that doesn't mean one should say that AC is PWM modulation.
Choosing the correct terminology for servo signals is complicated by the fact that many smart people (like westfw) do use the term PWM when talking about servos. But the term PWM for servo modulation can [u]confuse people[/u] into thinking that analogWrite is suitable for driving servos, so what should it be called?
It makes sense to use the same termonology used by the people designing and manufacturing RC electronics. And the major manufactures of RC equipment describe this modulation as PPM: http://www.futaba-rc.com/faq/product-faq.html#q1 http://www.hitecrcd.com/product/comparison_chart/26/Optic_Radio_Chart.pdf http://www.spektrumrc.com/DSM/Technology/glossary.aspx
Avoiding references to PWM controlling servos should help reduce confusion with the output from analogWrite, so I will continue to evangelize the use of PPM to refer to servo signals.