-The Erev Rav is the concept of attacking Yisroel from the inside.
-By becoming part of Yisroel they are able to cause great problems.
-The original Erev Rav came from the outside through the converts that left Egypt with Yisroel.
-These converts were mostly from all seventy nations rather then pure Egyptians. This is not surprising for Egypt at that time was the center of the world.
-It is for this reason that the Erev Rav has such a spectrum of powers for they draw from the roots of all seventy arch nations.
-Many of these Erev Rav were descendents from those people that Yosef converted. Converted over here means that they believed and followed the path of a single G-D.
-After the death of Yosef and the beginning of the enslavement many returned to their evil ways.
-This is one of the reasons Moshe agreed to take them for he knew that they must contain sparks of good for they had originally been involved with holiness.
-Not all the Erev Rav fell into this category for two of their biggest leaders were Yuli and Mamrie the two sons of Bilam.
-Also there were parts of Yisroel that already were affected by the concept of Erev Rav since they had greatly mingled with the Egyptian culture. Most of these were from the tribes of Shimon and Reuvain. There were parts of these tribes that were not enslaved and actually held government positions. Their leaders were Dassan and Avirum.
-This is also one of the reasons Yisroel had to travel through the long route in the desert, it was hoped that many of the Erev Rav would not be able to withstand the hardships and would turn back. This actually happened, many Erev Rav joined the Egyptians when they came to attack, there were also many that deserted as all the other hardships came up.

(Rabbi Yonasan, Tiferet Yonasan – Bshalach)