The switching element inside a Reconfigurable Optical Add-Drop Multiplexer (ROADM).
A WSS typically comes in a 1xN format, with one fiber going in and going out. One of the outgoing fibers is an express lane, for the DWDM wavelengths that aren't being added or dropped by this particular ROADM. The remaining fibers can be used to add or drop wavelengths -- so, a 1x9 WSS creates a ROADM that can add or drop 8 wavelengths.
Vendors are starting work on 1x23 WSSs, but the most commonly used types are still the small ones, as small as 1x2.
WSSs are available with 100GHz and 50GHz ITU grid spacing, and some components vendors say they can also go "gridless," using software to assign wavelengths that aren't confined to one ITU grid assignment. (See OFC/NFOEC: For ROADMs, Less Is More.)
JDSU and Finisar design their own WSSs. A handful of startups also deliver WSSs, including Capella Photonics Inc., CoAdna Photonics Inc., and Nistica.