Full Idea
Zermelo-Fraenkel and Gödel-Bernays set theory differ over the notions of ordinal construction and over the notion of class, among other things. Then there are optional axioms which can be attached, such as the axiom of choice and the axiom of infinity.
Gist of Idea
Major set theories differ in their axioms, and also over the additional axioms of choice and infinity
Source
Michèle Friend (Introducing the Philosophy of Mathematics [2007], 2.6)
Book Reference
Friend,Michèle: 'Introducing the Philosophy of Mathematics' [Acumen 2007], p.42
A Reaction
This summarises the reasons why we cannot just talk about 'set theory' as if it was a single concept. The philosophical interest I would take to be found in disentangling the ontological commitments of each version.