The document discusses different plug-in architectures including Emacs, Mozilla, and Eclipse. It provides an overview of how plug-ins work and are developed for each system. Some key points include: Emacs extensions are written in Lisp and can modify or replace core functionality. Mozilla uses XPCOM and XUL/JavaScript to build plug-ins, with XPCOM allowing more complex C++ extensions and XUL/JavaScript used for UI modifications. Eclipse is based on a small core that loads independent plug-in components written in Java.