DEPRECATION ALERT
Fedora encourages developers to discontinue defining the using RPM Groups tag within their spec files when building packages. CentOS5 and EL5 packages are still expected to have this defined, but that is not the case for building packages for Fedora and for CentOS and EL versions newer than v5. Leave it out of your RPM spec file. Reference: [1]
The rest of this document is maintained for posterity.
Valid (but deprecated) RPM GROUPS
Fedora (still, as of even version 25) lists acceptable package groups in /usr/share/doc/rpm/GROUPS (unchanged at least since Red Hat 9). They are:
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
In common use is another group that is not part of the "official list". It exists as a sort of non-group group:
Unspecified
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