Windows CE is an operating system developed by Microsoft developed with embedded systems in mind. It was originally developed for handheld computers, with the first release debuting in 1996.
The Sega Dreamcast is known for running an optimised version of Windows CE (with DirectX) as an operating system. It was developed between Microsoft and Sega over a period of two years, with the original intention of being the primary operating system of the console. It was Microsoft's first meaningful foray into the video game console market, which would continue with the Xbox a few years down the line.
In the end, Sega chose to go down a different route than what was originally planned - bundling simple operating systems within the discs themselves. Running games through Windows CE was still an option for developers, but most games used Sega's proprietary software instead. The limited adoption of Windows CE is often blamed on console programmers lacking the experience to deal with the OS, and a lack of advantages that Windows CE offered over the competition.