The Nature and Source of the Erev Rav Soul

QUESTION
What does it mean that the Erev Rav are in the Jewish people and they look like Jews but they do not have a Jewish soul? What is the source of this? And is this referring to the neshamah, or to the nefesh habehaimis?

ANSWER
Refer to the sefer HaDor Ha’Acharon which collects together many statements of the Sages on this matter.
The Erev Rav are the evil parts of the Nefesh (soul) of Moshe Rabbeinu. They came from the kelipas noga in Moshe’s soul, and this is the root of all bad middos in Klal Yisrael (as explained in the beginning of sefer Tanya). On the outside it is not recognizable who is from the Erev Rav, but this is what they are on their inside.