The ".d" directories are used like this:
Instead of adding text to one configuration file, e.g.
/etc/httpd/conf/httpd.conf, you add a file to the appropriate ".d"
directory (/etc/httpd/conf.d in this case).
It usually works just like if the files from the ".d" directory were
included verbatim in the main configuration file.
Why would you want to do it like that, instead of just adding your
stuff to one global configuration file? It is much simpler and cleaner
when installing packages - for example from an .rpm or .deb file. The
installer doesn't have to parse the configuration file to see in which
place to add the needed lines. It just places a file in the ".d"
directory. And when you remove the package, the file is removed -
imagine the pain if you tried to remove the right lines from the
global configuration file.
The scheme is used by many applications: cron, xinetd, PHP, PAM, even
by the shells (/etc/profile.d).
I don't know what the letter ".d" stands for.
Hope this helps,