â„–18875150[Quote]
In set theory, a "set" is a collection of objects that is itself an object and can be an element of other collections. A "proper class" is a collection that is defined by a property, like "the class of all sets," but it is too large to be a set. If you tried to treat the collection of all sets as a set, you'd run into paradoxes like Russell’s Paradox. So, we distinguish between them to keep the system consistent—sets are the "building blocks" that can be members of other collections, while proper classes are collections that can contain sets but cannot be members of anything themselves.