It depends on how you look at Judaism. You could look at it like a particular set of descendants (race), a collection of cultural practices (ethnicity), or you could focus on the beliefs of the people (religion).
The lines between those categories are blurry. One may convert to Judaism, and be recognized as Jewish by the larger community that way. There are also those who have joined into Jewish communities throughout time and so modern Jewish communities contain people from a plethora of descendants.
The lines between those categories are blurry. One may convert to Judaism, and be recognized as Jewish by the larger community that way. There are also those who have joined into Jewish communities throughout time and so modern Jewish communities contain people from a plethora of descendants.