Most of the functionality that dracut implements are actually implemented
by dracut modules.  Dracut modules live in modules.d, and have the following
structure:

dracut_install_dir/modules.d/
	00modname/
		install
		check
		<other files as needed by the hook>

00modname: The name of the module prefixed by a two-digit numeric sort code.
	   The numeric code must be present and in the range of 00 - 99.
	   Modules with lower numbers are installed first.  This is important
	   because the dracut install functions (which install files onto
	   the initrd) refuse to overwrite already installed files. This makes 
	   it easy for an earlier module to override the functionality of a 
	   later module, so that you can have a distro or system specific
	   module override or modify the functionality of a generic module
	   without having to patch the mode generic module.

install: dracut sources this script to install the functionality that a 
	 module implements onto the initrd.  For the most part, this amounts
	 to copying files from the host system onto the initrd in a controlled
	 manner.  dracut supplies several install functions that are
	 specialized for different file types.  Browse through dracut-functions
	 fore more details.  dracut also provides a $moddir variable if you
	 need to install a file from the module directory, such as an initrd
	 hook, a udev rule, or a specialized executable.

check: Dracut calls this program to check and see if a module can be installed
       on the initrd.
       
       When called without options, check should check to make sure that
       any files it needs to install into the initrd from the host system
       are present.  It should exit with a 0 if they are, and a 1 if they are
       not.

       When called with -H, it should perform the same check that it would
       without any options, and it should also check to see if the 
       functionality the module implements is being used on the host system.
       For example, if this module handles installing support for LUKS
       encrypted volumes, it should return 0 if all the tools to handle
       encrpted volumes are available and the host system has the root
       partition on an encrypted volume, 1 otherwise.

       Check may take additional options in the future.  
       We will most likely grow a module dependency checking system in the
       near future, and check will need to handle a -d option when we do.

Any other files in the module will not be touched by dracut directly. 

You are encouraged to provide a README that descrobes what the module is for.