Some years ago Dan Burcaw at Yellowdog Linux (an RPM-based distribution targeted at Apple Macintoshes) together with Bryan Stillwell, Stephen Edie, and Troy Bengegerdes created the Yellowdog Updater (yup). Yup extracted dependency information from RPM headers and constructed full dependency trees, permitting the automation of RPM installation and maintenance. On the strength of its features, Duke University (especially the Physics Department) came to rely heavily on yup.
However, yup was extremely slow. Operating on the client side, it had to completely download RPMs in order to extract critical header information to resolve all possible dependency loops. The header is only a tiny fraction of most RPMs so this was obviously very inefficient. Powerful as it was, yup was also missing lots of obviously desirable features.
As a GPL project it was also easy to get involved and work to improve it. Seth Vidal, the Duke University Physics Department's highly talented systems administrator, took on this task. At first Seth worked to improve yup, but soon it became apparent that the tool would have to be completely rewritten in order to make it faster and more powerful, so at some point he split off from yup and created yum, the ``yellowdog updater, modified''. The resulting tool was an instant hit.
Seth was later joined by a physics post-doc, Michael Stenner, who took on the development of a generalized ``urlgrabber'' that manages most of yum's remote file access transactions, and many others (mostly systems administrators of large networks and beowulf cluster that now use yum as a primary maintenance tool - see the list of contributors on the yum project page for a complete listing).
With active input from many highly talented administrators and a very energetic open source development team, yum has become a very powerful tool indeed, capable of working such dark magic as safely upgrading a system from one distribution release to the next while the system is operating, as well as installing packages, updating packages, providing information about installed and available packages, and much more, all while operating many times faster than yup was able to do.
At this point, yum is supported and distributed via DULUG's public web resources and the Linux@Duke project (see links above).