A YUM group is a set of RPM packages collected together for a specific purpose. For example, you might collect a set of "Development Tools” together as a YUM group.
A group is specified by adding a group XML file to same directory as the RPM packages included in it. The group file contains the metadata of the group including pointers to all the RPM files that make up the group.
A group file can also be created by running the following command:
sudo yum-groups-manager -n "My Group" --id=mygroup --save=mygroups.xml --mandatory yum glibc rpm