QNX is a POSIX-compliant real-time operating system that includes its own GUI, called Photon. (Support for X-Windows is optional.) Programming for QNX is similar to programming for any other POSIX system, and many programs written for Linux can be compiled under QNX without changes.
QNX was designed to be highly modular, allowing system designers to only include those portions needed and leaving the rest out. The most extreme example of this was the QNX demo disk, a single-floppy version of QNX 4 that included a GUI, a web browser, a TCP/IP stack, either a generic network card driver or a generic modem driver, and a few other programs.