A collection of PHP files
It has added features like turning on and off displays for things like directories, files, file extensions and more.
You can also select what filetypes you want only to see within your directory.
It has some support for skinning (with CSS class names for most elements)
And the index file is small (only 4 lines of actual code), so its very easy to have the index file generated by another program.
There is also support for translation via the lang.php and config.php files.