/etc/portage/sets

For each file in, a package set is created, defining a one-to-many relationship between a set name and an arbitrary group of package atoms. User-defined set names correspond to the name of the file. Each file should contain a list of package atoms and nested package sets, one per line. You can see the full list of sets that exist at any time with

Example
Say you were running live ebuilds for GPU drivers, and wanted an easy shorthand for rebuilding them on demand. Define this set:

Now, all that is needed trigger a rebuild of all of them at once is:

When you want to remove a user defined set from the world_sets, use the  option to emerge, then clean up orphanded dependencies:

External resources
For more detailed information about package sets, read the emerge manpage and search for "set" ( /^\s*set ).