Well first of all they are the children of Angles that were corrupted, fallen angles, and thus corrupted them selves. We really aren't clear about much that happend before the flood aside from things that are written down. The Nephilim are the products of fallen angles and human women.
Genesis 6:4 "The Nephilim were on the earth in those days—and also afterward—when the sons of God went in to the daughters of humans, who bore children to them. These were the heroes that were of old, warriors of renown."
As we can see the "sons of God" (Angles) who had children (Nephilim) with human women. These Nephilim became the heros of old, warriors of renown.
These Nephilim are probably the basis for the ledgendary Gods of the past, and the ledgends of them were probably the building blocks for ledgends like Herculies and other myths.
That is the best I can come up with, from what I have read. As to where they are, well to be honest 2 possibilities exist.
1. They are all dead.
2. They live some where beyond earth or maybe even beyond this dimension.
Number 1 is most likely in my opion. One of you might be wondering then why do the y appear again after the flood as quoted in:
Numbers 13:33 "And there we saw the Nephilim, the sons of Anak, who come of the Nephilim: and we were in our own sight as grasshoppers, and so we were in their sight."
Here is probably a different group of Nephilim as the others are most likely dead. Since it is the word giant and not the name of a specific group of people, that they use here. Also these Nephilim are "the sons of Anak" a term that not connected with the first group of Nephilim. These 2 points lead me to believe that they are 2 seperate groups of Nephilim. Remember these are the words of the frightened Jews, who took the old ledgends are tried to scare them away from the land.
Numbers 13:32 "And they brought up an evil report of the land which they had spied out unto the children of Israel, saying The land, through which we have gone to spy it out, is a land that eateth up the inhabitants thereof; and all the people that we saw in it are men of great stature."
So most likely the Jewish search party were exagerating this second group to make them appear to be the first to scare there fellow Jews even more.