The X11 Inter-Client Communications Conventions Manual [L1 ]
"The ICCCM is generally the M in "RTFM" and is the most-important of the least-read X documents."
This document specifies how X clients interact with each other. It explains how to use the X selection mechanism, how to interact with the window manager (and what a window manager is), session management, and how to manage shared resources like the keyboard focus and colormaps.
The Tk toolkit handles most of the ICCCM conventions internally, so as an application writer you don't need to worry about it too much. It's good reading though if you want to know what's really going on with the clipboard or what happens when you execute selection get.