httpd.conf file is Apache server configuration file. In that file you can configure Apache server settings. You can to install Apache HTTP server in many OSes. And I ever install it on Mac OS X and Windows XP. And Apache doesn't provide a GUI configuration screen so you must find the httpd.conf file and then open it with text editor application such as notepad to configure it.

On Windows XP you can find the httpd.conf file in conf folder inside installed Apache folder (if you installed on its default folder so you can find it in : C:\Apache Software Foundation\Apache2.2\conf\ folder.
And on Mac OS X you can find the httpd.conf file inside folder: /etc/httpd/folder.