Mac OS X makes extensive use of xml files; xml is a structured language closely related to html.
TeXShop uses five xml files for configuration: completion.plist, autocompletion.plist, KeyEquivalents.plist, Macros_Latex.plist, and Macros_Context.plist. These files reside inside your personal library in a folder called ~/Library/TeXShop created when TeXShop first runs. If you have modified the default TeXShop configuration and want to move TeXShop to another machine preserving your modifications, copy ~/Library/TeXShop to the new machine.
The files are used to configure the Latex Panel, Auto Completion, the Keyboard Menu Shortcuts, and the Macro menu. Details are given elsewhere in this document.
Files of type plist are ordinary text files, so they can be opened and edited with TeXShop, TextEdit, or other text editors. Editing the file is straightforward, but somewhat tedious.
If a plist file contains unicode characters, it needs to be edited and saved in UTF-8 format. Before opening such a file in TeXShop, change the TeXShop encoding preference to UTF-8. Then edit and save the file. Then change the encoding preference back to the original value. The default value is MacOSRoman if you did not reset it earlier.
If you installed the Developer distribution of Mac OS X and you double click a .plist file, it will open in a program named Property List Editor. This program is useful for editing plist files, but it is buggy and does not display all file information. In particular, comments are missing. It is better to use TeXShop.