What is the difference between
yum update and
yum upgrade, and when should I use one over the other?
yum upgrade forces the removal of obsolete packages, while
yum update may or may not also do this. The removal of obsolete packages can be risky, as it may remove packages that you use.
yum update the safer option.
If run without any packages, update will update every currently installed package. If one or more packages or package globs are specified, Yum will only update the listed packages. While updating packages, yum will ensure that all dependencies are satisfied. (See Specifying package names for more information) If the packages or globs specified match to packages which are not currently installed then update will not install them. update operates on groups, files, provides and filelists just like the "install" command. If the main obsoletes configure option is true (default) or the --obsoletes flag is present yum will include package obsoletes in its calculations - this makes it better for distro-version changes, for example: upgrading from somelinux 8.0 to somelinux 9.
Is the same as the update command with the --obsoletes flag set. See update for more details.