Sets are collections of arbitrary objects, considered as an object in its own right, e.g. * your family (a collection of all you relatives) * a book (as a collection of sheets) * me and you ("us") it's not even necessary for the set to be a particular meaningfull collection, nor to contain any object at all: are all perfect sets. The things that make up a set are said to be its "elements". So, * your father is an element of your family * i am an element of us * the sun is an element of me, you, the number 1 and the stars ('cause the sun itself is a star) example