Georg Ferdinand Ludwig Philipp Cantor is considered the creator of set theory, and his theories are the basis for the naïve set theory you learned in school. But there are lots of other mathematicians you should know, such as Hilbert, Frege, Russell, Zermelo and Dedekind. They made a lot of contributions, too.
Hilbert Hilbert is best known for asking the first questions about infinities – all the mathematicians want to stay at “Hilbert’s hotel” and you can look up some YouTube video animations that explain his work. Dedekind is responsible for “Dedekind cuts” which give a good consistent model of