The student is assumed to have a working knowledge of the elementary theory of sets. More specifically, he is expected to know the algebra of sets and the most basic facts concerning cardinal and ordinal numbers, and to be able to follov and use arguments based on the axiom of choice, the veil-ordering principle, or the various maximal principles related to the axiom of choice. The approach to set theory will be informal, and no particular system of axioms need be specified.
It is important to be aware of the distinction between the concepts of a class and of a set, sets being by definition classes that are members of other classes.
Actually this distinction will occasionally be disregarded, but only when this is not likely to lead to the familiar set-theoretic paradoxes, and the abuses could in any case be avoided by reformulating the discussion in a more rigorous but less suggestive language.