Windows CE is the base OS: a modular, component, real-time embedded OS. Windows Mobile is an implementation of it, as is the OS embedded in a SPOT watch, the Windows Vista Slideshow*, the ill-fated Smart Display, a Windows CE sewing machine, robot, KiSS networked DVD player or SEGA Dreamcast. See a few more devices here: WindowsForDevices. Partners take the base OS, build a platform from it and feed their changes back into the next cycle of CE; you can also get the source code and play about with it - it's used by a number of universities as a learning tool.
*that's the external display not the Sideshow sidebar
