The CLIM core, comprised of sheets, mediums, graphics, and input and output, serves as the foundation for higher-level functionality. CLIM itself provides many advanced capabilities that have been developed from this kernel. Presentations, streams, and gadgets are all descendants of the fundamental CLIM kernel. This resulting hierarchy of objects and functionality gives CLIM a layered structure. For instance, we notice that streams and gadgets are specialized panes that are themselves specialized sheets. Similarly, presentations are customized output records. At any point in these hierarchies, one may customize and specialize objects by making subclasses of existing objects and adding the desired functionality. Although CLIM provides many advanced facilities, it is always possible to return to the fundamental CLIM building blocks and start creating anew.
Common Lisp Interface Manager 2.0 User's Guide - 20 Sep 2011