httpd.conf is the configuration file for the apache webserver. If you're using Apache 1.x, it's located at /usr/local/etc/apache/httpd.conf. If you're using Apache 2.x, it's located at /usr/local/etc/apache2/httpd.conf.
There should probably be a LOT more said about this file. Most of it should probably be said in a Configuring Apache article instead of here.