/etc/portage/sets

is an optional directory that that is used to create user defined package 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
For the sake of example, say Larry wanted to run live ebuilds for GPU drivers, and desired an easy shorthand for rebuilding them on demand. He could define the following set:

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

To remove a user defined set from, use the  option to emerge, then clean up orphaned dependencies:

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