To create a Group or Group Set:
If you remove a reference from a group it will remain in your library and can be found in All References. You can put a reference in more than one Group, which will not duplicate the reference.
To see which Group a reference is in, right-click on the reference, or go to the References menu, and select ‘record summary’.
Some temporary Groups are automatically created when you carry out certain functions in EndNote:
This video (3 mins 16 secs) covers:
A Smart Group is dynamically populated with references that fulfill your specified search criteria as you add references to and edit references in the library. You could, for example, add tags to a specified field such as the keywords field, and create a Smart Group that includes all references that have that particular tag.