It is very simple indeed to set up a yum repository. There are only three steps. We will briefly outline them below, making the assumption that you are going to set up a mirror of the Fedora core repository on the DULUG (Duke University Linux User's Group) public Red Hat Mirror:
http://mirror.dulug.duke.edu/pub/fedora/linux/core/1/i386/os/
Note that this path (and the public accessibility of this server) are both subject to change. Modify the path and scripts below to mirror the particular repository of your choice, or create your own repository from (for example) CD images for your favorite RPM-based Linux distribution. Note Well however, that if you create a ``static'' repository directly from CD/ISO images, yum will not be able to perform one of its very important functions - managing package updates to keep your system current and secure. In this case you might consider using your static repository for a initial installs and "package shopping", but put additional repositories to provide you with updates into the yum.conf on your clients (see the section below on ``layering yum'').
Note also that if you are not interested in setting up your own repository, yum will still function as a client on your workstation(s) and server(s) if you can find a public repository that supports yum. Many of them do, in particular the Fedora repositories. If this describes you, you can skip all of this section except the part on getting yum.
A reasonably complete list of public repositories is available on the yum project site. Systems administrators caring for large numbers of systems are strongly encouraged to set up their own local mirrors of these repositories to (and if possible to make them publicly accessible) to minimize the load on the mirrors.