In the introduction of Hungerford's Algebra (p. 2), he gives a rather trivial example of a class that is not a set, but what is the purpose of even having this term defined? Is it useful, other than to give a name to collections of objects that are not sets?
Also, how is this term related to equivalence and congruence classes? More specifically, are there equivalence or congruence classes that are not sets?
EDIT: It turns out the "rather trivial example" is Russell's paradox and wasn't so trivial at the time of it's discovery.