A set is a collection of objects or components. Any item you can select can be in a set. The set exists as a separate node representing the collection. Unlike groups, sets do not alter the hierarchy of the scene - they're simply an arbitrary collection. They are always saved at the scene level, and cannot be part of an object, group, or hierarchy.
You can create two different types of custom sets: sets and quick select sets. Both of them can contain selected objects, components, or groups, but a quick select set can't be added to a partition. As their name implies, quick select sets are most useful for making it convenient to select any collection of objects or components. For example, you can select many vertices on a polygon object and put them in a set so that you can select and model with them easily.
In some instances, Maya creates sets for you as you work with objects.
Sets are useful for the following:
You can control the membership of sets easily using either the Outliner or the Relationship Editor, as described in Create and edit sets.
A partition is a collection of related sets. Partitions prevent the sets in them from having any overlapping members. Maya uses partitions to keep sets separate where overlapping members could cause problems.
You can create your own partitions when you want to create sets that have no overlap.