Yum Package Manager has been replaced by DNF Package Manager since many long-standing issues in Yum remain unresolved. Yum is a front-end tool for rpm that manages dependencies and repositories and then uses RPM to install, download and remove packages. The major differences between YUM and RPM are that yum knows how to resolve dependencies and can source these additional packages when doing its work. Though rpm can alert you to these dependencies, it is unable to source additional packages. As to installing vs. upgrading.
