The X Window System, often simply called X, is a graphical windowing system for bitmap displays. It provides the basic framework for building graphical user interfaces on Unix and Linux operating systems. X allows multiple applications to run simultaneously in separate windows, enabling users to interact with them visually. X operates on a client-server model, where the server manages the display and input devices, while clients are applications that request to draw on the screen. This architecture allows for flexibility, as clients can run on different machines while displaying their output on a single screen, facilitating remote graphical sessions.