You can find links for the editor floating around here somewhere, but dophin created a good centralized site for all this stuff:
http://www.nfl2k5rosters.com/
You can find a download for the editor, various rosters from a lot of the guys here, etc. Good resources.
Essentially the editor allows for updated rosters, expanded ratings ranges (if desired), adjustments to salary cap, coaching changes, unlocking of some player features, etc.
The editor itself has most of the info you need to use it. There are also some great tutorials around this forum as well.
Enjoy!