From Fedora Project Wiki

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