From Fedora Project Wiki
Valid RPM GROUPS
Fedora 22 lists them in /usr/share/doc/rpm/GROUPS (unchanged at least since Red Hat 9):
Amusements/Games Amusements/Graphics Applications/Archiving Applications/Communications Applications/Databases Applications/Editors Applications/Emulators Applications/Engineering Applications/File Applications/Internet Applications/Multimedia Applications/Productivity Applications/Publishing Applications/System Applications/Text Development/Debuggers Development/Languages Development/Libraries Development/System Development/Tools Documentation System Environment/Base System Environment/Daemons System Environment/Kernel System Environment/Libraries System Environment/Shells User Interface/Desktops User Interface/X User Interface/X Hardware Support
A useful command to show what groups are being used by a set of packages:
rpm -qa --qf '%{group}\n' | sort -u
Here's a variation useful to package developers seeking examples for each group. It lists every package installed on a system, sorted by RPM Group:
rpm -qa --qf '%{group} %{name}\n' | sort -u