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1: You get an OA.  You don't get CA, why would you?

2: Same as 1.

3: Yes, exactly as you quoted, being Prone grants a +2 to defences against nonadjacent ranged (NOT area) attacks.

4: They are totally different things is the difference.

Being Prone is lying down (or equivalent, for a snake it might be turned on its back, for example).  Prone creatures grant combat advantage for melee attacks made against them, and gain a +2 to defences versus nonadjacent ranged attacks.  They can only move by crawling or teleporting.  They can stand up as a move action.  It's a minor action to drop prone if you want to do so, but this imposes all the above penalties (and benefits).

Granting Combat Advantage happens under any number of circumstances (Prone, as above, dazed, stunned, due to flanking, etc etc).  Attacks made with CA get a +2 bonus to hit, and many in-game effects require it to trigger.  It's as simple as that.

Opportunity attacks happen when a creature leaves a threatened space (normally adjacent to an enemy, unless threatening reach is involved, but there are numerous specific exceptions.), or uses a ranged or area effect in a threatened space.  Standing up from Prone does not normally incur opportunity attacks.

It's worth noting, here, that Opportunity Attack is the 4e term.  Attack of Opportunity is the 3e term.  Also that the rules do what they say they do, no more, no less.  If something doesn't say you get CA, you don't.  If it doesn't say you're Prone, you're not.  Etc etc.

Feb 26, 2013 -- 9:52AM, dfn77 wrote:

1) When an adjacent enemy moves away (not shifts), do I get combat advantage AND an attack of opportunity (assuming enemy + me are NOT under any effects/conditions)?

Just the opportunity attack.

2) same as 1), but when an adjacent enemy uses a ranged/thrown weapon or area affect?

same as 1), just the OA.

3) For prone: "You get a +2 bonus to all defenses against ranged attacks from nonadjacent enemies." So when I am prone, I get a bonus to defense (vs ranged)?! Just want to make sure

Yes.  The rationale is that prone target presents a smaller profile to a ranged attacker, lots of games, particularly those with modern settings and gunplay, have a rule like this.

4) Essentially, maybe all of the above can be summed up as "what is the difference between being prone, granting combat advantage, and attacks of opportunity?"

Prone is a condition.  When you're prone, you can only crawl as a form of movement, you suffer an attack penalty, you are a bit harder to hit from a distance, and you grant combat advantage.  Combat Advantage is any sort of advantage in combat - flanking, prone, unable to see an attacker, etc - and it doesn't stack from multiple sources, so if you're flanking a prone, blind, dazed, enemy, it's still just +2.  An opportunity attack is an out-of-turn action provoked by an adjacent enemy with whom you're fighting.  The idea is that some actions leave a creature in melee open to attack, trying to use a bow or cast a fireball when a barbarian is in your face swinging a sword, for instance, likely gets you hit by that sword.