In a nutshell, a Silent Wave Motor (SWM) is a motor installed in the Nikon lens which helps focus faster and without making appreciable noise.
Not all Nikon cameras have an autofocus motor within the camera body. For example, D3200 does not have an autofocus motor and therefore a lens having an auto focus motor has to be bought. An AF-S (Auto Focus Silent) lens has a Silent wave motor. Any lens having AF-S employs this technology.
The Silent Wave motor is a specific application of a generic technology known as an Ultrasonic Motor. Without getting into the actual physics, an SWM essentially converts travelling waves or vibrations into rotational energy to rotate and hence focus the optics in the lens. This results in smooth, quiet and faster auto-focusing. Clearly, this is very useful when shooting sports or fast moving subjects (where the camera needs to focus quickly). It is also useful in wildlife photography since it makes very little noise. The Ai AF-S NIKKOR 300mm f/2.8D IF-ED, released in 1996, was among the first lenses which which had SWM. This superseded AF-I (auto focus internal) which had a regular electromagnetic motor as the autofocus system in the lens.
SWM has been adopted by almost all the NIKKOR lenses being manufactured. Please note that although these are quieter they can still make slight noises while functioning. It can be caught on the built-in microphone while a video recording. Therefore it is not recommended for professional videography.
There can be further two types of Silent Wave Motors. One is the Ring SWM and the other is Micromotor SWM. In the Ring SWM there are metal rings within the motor which vibrate at a high frequency. These wave transfer energy without even though there is no moving part like in a conventional motor. These lenses are truly fast to focus and make very little noise. Ring SWM's are expensive as compared to Micromotor SWM. All pro grade lens have this motor.
A Micromotor SWM on the other hand looks more like a conventional electric motor with gears and a pinion/tooth to focus the lens system. Most of the entry level lenses employ this. These motors can make a slight whirring sound and take longer to focus.
Some popular lenses having SWM:
AF-S 17-35mm f/2.8D IF-ED
Nikon AF-S DX Zoom-Nikkor 55-200mm f/4-5.6G
Nikon 18-200mm f/3.5-5.6 G AF-S VR DX Nikkor
Each major lens manufacturer has employed a version of WSM in their lenses, and they all call it something different. For example:
Canon – Ultra Sonic Motor (USM) . Canon was the pioneer in this field.
Sigma – Hyper Sonic Motor (HSM)
Tamron – Ultra Silent Drive (USD)
Minolta, Sony- Super Sonic Motor (SSM)
Olympus – Supersonic Wave Drive (SWD)
Pentax – Supersonic Dynamic Motor (SDM)
Panasonic – Extra Silent Motor (XSM)
Tokina – Silent Drive Module (SDM)
Despite a slight variation in the naming, all work on the same principles. I hope this has cleared the mystery behind the tag AF-S on your favourite lenses!
References:
Read more on NikonRumors.com: https://nikonrumors.com/2012/06/14/announcment-day-70-million-nikkor-lenses-produced.aspx/#ixzz378ga0sCK
https://en.wikipedia.org/wiki/Ultrasonic_motor