Yum is a tool designed to provide a user or systems administrator with the ability to fully automate all aspects of Red Hat Package Manager (RPM) package installation and administration on RPM-based systems (both Linux and otherwise). It can be viewed as ``competing'' with package management tools such as Red Hat's up2date, Debian's Advanced Packaging Tool (apt-get), Mandrake's utility for managing and installing RPMs, (urpmi) and apt4rpm. If you like, its primary purpose is to prevent "dynamic link-library hell" on RPM-based systems while providing users and administrators a much higher level interface to RPM packages and repositories than that provided in the basic RPM .
Yum, however, is not YAAPT (yet another advanced packaging tool). Package management is far from trivial, and each of these tools has been driven by a distinct philosophy. Yum has from its post-yup beginnings been designed and improved strictly by experienced systems administrators managing large networks of systems. Its development is non-corporate and distribution agnostic, and as a client pull tool that works from package repositories it supports a decentralized management model that fits well in the Universities, government labs, and cluster environments whose administrators have primarily contributed to its development.
Yum is still a relatively young tool - it lacks a graphical user interface (GUI) for example, because its primary application in administered networks really doesn't need one - but is quite feature mature and is very portable. It is also under extremely active development, with features planned that will make it even faster and more efficient.