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 object representing the collection. Unlike groups, sets do not alter the hierarchy of the scene.
In some instances, Maya creates sets for you as you work with objects. For example, when you add a cluster deformer to some CVs of a NURBS surface, Maya makes a set for the CVs. You can edit the set to control the effect of the deformation. Maya also creates sets that represent shading groups and layers, and points controlled by deformers, flexors, and skin.
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.
For example, suppose you’re animating a cartoon character’s nose as he smiles and laughs. You added a cluster to several CVs for adjusting the nose as he smiles and another cluster to different CVs for adjusting the nose as he laughs.
Creating the two clusters creates a set for each group of CVs. Occasionally you want to move CVs from one set to the other. When you move the CVs from one set to the other set, they remain in the first set. You might not want the CVs in the first set because they add undesirable deformations as you transform the cluster.
To avoid this problem, you can create a partition and put both sets in it. The partition prevents one set from having members of another set. When you move the CVs from the first set to the second set, they’re automatically removed from the first set.