I think I can help a little:
Rangers:
The Ranger School is open to all combat MOS's and most officer candidates. All who graduate wear the Ranger Tab, no matter what unit they are assigned to, over the unit patch, left shoulder.
Then, there is an actual Ranger Unit, the 75th Ranger Regiment, which has it's three combat battalions located in three seperate locations. Every member attends Ranger School (better sooner than later) and they wear the Ranger Tab over their unit patch, the Ranger Scroll, which indicates which "bat" they belong to.
A Ranger tabbed soldier in a regular unit is basically "the shit". It commands respect.
Special Forces units are very confusing to keep up with what each ODA does and is capable of, so I won't try to act like I know. Some are scuba specialist, some HALO, etc. All have different areas of the world that they specialize in.
As for Delta, well, they don't exist, now do they?