What is the full scope of this clause? does it just refer to privateering (state sanctioned piracy) on the high seas? Are letters of Marque and letters of Reprisal two different things (and why haven't i heard about the second?) or does it all refer to one thing?

letters of marque and reprisal were extremely common in the age of sail. in an era with relatively weak states and (relatively) inexpensive ships, it was an absolutely integral part of most countries' naval strategy to use privateers to disrupt enemy commerce. fyi, don't ever conflate "privateering" with "piracy," because the two are totally legally distinct.

the granting of a letter of marque means that the holder is acting on behalf of the holder's government. under the laws of war at the time, a privateer was a warship acting under a well-established legal regime. the letter of marque limits the privateer to attacking enemy ships, and any vessel captured would have to be taken back to port for an admiralty court to decide how much prize money the privateer would make from the capture. for example: if you're an american privateer, and america is at war with britain, you can only attack british ships-- and any captured vessels have to be returned to american courts to claim your reward. jean and pierre lafitte, american heroes of the battle of new orleans in 1815, fell into this category.

pirates, on the other hand, were the enemies of all, answered to no government and no law, and were liable to being chased down and summarily executed if ever caught.

this line often was blurred during the anarchy of latin american wars of independence, however-- it was easy to fake a letter of marque from one of the many of new latin american states. (this ultimately led to the lafitte brothers' downfall, when it was discovered that jean lafitte was sailing under a forged letter of marque.) see william davis, the pirates lafitte.

What is the history of the use of these letters? i know they were used in 1812 and the am revolutionary war but i know no specifics. Were they used after this? have they been outlawed by international treaties and across the world when did this process die out?

there are two reasons why they died out. first, as warship technology advanced, it became much more difficult and expensive for privately-funded commerce raiders to be effective. in 1775, it was a quite feasible to convert a merchantman or an obsolescent warship into a serviceable, profitable commerce raider, but by 1861 it was no longer profitable. the confederate states learned this the hard way. because privateers had to bring their prizes to a court to get paid, confederate privateers had to run the federal blockade to get their prizes in, and run the same blockade to escape once again. blockade-running is dangerous business, and ordinary merchantmen (which were the usual types of prizes) were too slow and too big to be effective. making matters worse, because privateers were lightly armed and designed for speed, they were generally no match for federal warships when it came to a fight. (for that matter, purpose-built confederate naval cruisers were no match for federal warships. the uss kearsarge blasted apart the css alabama in the battle of cherbourg in 1864 and it wasn't even close.)

second, as a legal matter, the great powers of europe adopted the paris declaration in 1856, outlawing privateering. the united states never became party to the paris declaration, but the federal government during the civil war agreed to abide by its principles. see james mcpherson, war on the waters.

did letters of marque and reprisal come up in the debate over iran contra/would it apply?

no. the u.s. federal government has adopted the principles of the paris declaration even though it isn't a signatory, and it hasn't issued letters of marque since before the civil war.

the internet (quick googling) tells me the us considered issuing letters of Marque during andrew Jackson's presidency. Why?

this i have no idea about. there was no war going on during jackson's presidency.

I don't know as much as I'd like about their use in the U.S. specifically, but I can answer some of your questions:

What is the full scope of this clause? does it just refer to privateering (state sanctioned piracy) on the high seas?

Privateering is what is covered, yes. The idea is that the federal Congress is the only group that can issue letters of marque or reprisal, which are two types of what, for simplicty's sake, we'd call legalized piracy.

Are letters of Marque and letters of Reprisal two different things (and why haven't i heard about the second?) or does it all refer to one thing?

They are somewhat different things, although usually a single document would be called "a letter of marque and reprisal." I can't speak to why you haven't heard of the second, but a brief explanation of the differences is:

• A letter of marque authorizes a private ship's captain and crew to attack enemy ships in time of war, whether civilian or military.

• A letter of reprisal is somewhat distinct, in that it allows a private citizen to capture property in reprisal for an injury done to them (in other words, let's say a Spaniard captures some of my wine; I can then apply for a letter of reprisal and legally capture equivalent goods from a Spanish ship, not necessarily the one that captured my stuff).

i know they were used in 1812 and the am revolutionary war but i know no specifics. Were they used after this?

The US did not issue letters of marque after 1815.

have they been outlawed by international treaties and across the world when did this process die out?

The Paris Declaration Respecting Maritime Law of 1856 abolished using letters of marque. They were briefly brought up again in the U.S. after the 9/11 terrorist attacks, but discussion of that won't be appropriate here due to our 20-year-rule.