These may not specifically be what you're looking for, but hopefully they'll provide a good jumping off point for discussion.
John Norton's
Breaking Down Defenses is a very good general overview of defensive responsibilities by position. My
Guide to NFL Defenses goes into some of those differences in greater detail. I believe both are available to non-subscribers currently. If not, you can access the precursor threads to my article through the links in my signature (there are separate discussion threads for both
3-4 and
4-3 fronts).
If you'd like more specific details than what may be found in the above links about each position's responsibility (who's usually tasked with X, Y, or Z general assignment). how the assignments differ in certain schemes or playbook terminology (who's called what and why), post them here and I'll see what else I can find or answer.
Hope that helps.