Sentence:  In the present section we shall discuss the various systems of set theory which admit, beside sets, also <WSD>classes</WSD>. <WSD>Classes</WSD> are like sets, except that they can be very comprehensive; an extreme example of a <WSD>class</WSD> is the <WSD>class</WSD> which contains all sets....The main point which will, in our opinion, emerge from this analysis is that set theory with <WSD>classes</WSD> and set theory with sets only are not two separate theories; they are, essentially, different formulations of the same underlying theory. 
Meaning of the word marked with <WSD> tags:  (Q217594)  A collection of sets definable by a shared property.