There is tons and tons of information that can be doled out in regards to Special Operations and/or black ops groups, Cirro... You'll likely find more real world info on the various Special Ops groups, though, as Black Ops, per the definition of the term, are just that - 'they never happened', 'they don't exist', etc - buried under mountains of misdirection, misinformation, etc, and never 'directly' alluded to in any way, shape, or form.
When it comes to Special Ops and such, though, you'll find a lot of 'general information' on any number of groups (and, in some places, even references to their tactics, etc). To answer the easiest of the questions that you started this thread with, however, the difference between an Army Ranger and your standard Army grunt is, quite honestly, training, cross training, and more training.
A couple of things to run through Google (or your search engine of choice) in regards to the topic are:
Force Recon (USMC "special forces")
Delta Force (this is where you might get into some more of the Black Ops kind of stuff - "Delta Force" (as it's called) is a 'multi service' special ops group that really doesn't have a name and, officially doesn't exist)
Army Rangers (of course) - Don't forget to also search on stuff like 'Green Berets'.
SAS ('Special Air Service
s' (thanks Ginafae) if I remember correctly. Basically UK bad-a$$ commandos)
The list goes on and on with the Soviet Spetznaz (sp?), etc, etc, etc... Might also want to take a trip to the local Library and pick up a book called 'Secret Armies' (can't think of the author's name, ATM, but will check into that and post an update ASAP).
Anyhoo... if you've got specific questions, etc, just throw them out there... there's plenty of military folks in our community and, likely, one or more with some law enforcement insight... Heck, I might have a shadowy secret or two, myself...
P.S. Moved to the Common Room... more likely to get noticed as an "open discussion" there.
P.P.S You moved it here while I was trying to move it here, and I moved another topic as a result...Gah! - Grugg