Many Linux installations (my favorite is Fedora though I'm not sure why) use yum to update their program files and libraries. During recent installation attempts with Fedora 14, I kept getting error messages saying repomd.xml could not be found which is a very important file used by yum to perform it's upgrades. I discovered that this file is normally found in a directory called repodata off the base directory provided. So if you are trying to provide the location for this file and it is located on an HTTP server or FTP server, you would specify the base url as http://host/os where the os directory on the host has a folder named repodata which contains repomd.xml
I will be making several entries regarding my Fedora 14 install. I've been at it for over 24 hours and it has been a terrifically difficult process.