Disclosures about the National Security Agency's spying on U.S. allies, including France and Germany, have sparked outrage in Europe and created tensions in trans-Atlantic relations. But just how widespread is such spying? Here are four things to know.

1. Who spies on whom?

Spying on adversaries is common — as is spying on your allies.

As Charles Kupchan, a professor of International Affairs at Georgetown University and a senior fellow on the Council on Foreign Relations, told NPR's Audie Cornish last week: "Everybody spies on everybody, including friends on friends."

But there are some exceptions. Since World War II, the U.S. and Britain have shared sensitive intelligence. Three other countries belong to this so-called "five eyes" alliance: Australia, Canada and New Zealand. They probably don't spy on each other's leaders, but their citizens are fair game.

"In fact," writes Max Boot in Commentary, "this intelligence sharing allows them to do an end-run around prohibitions on domestic surveillance: the Brits can spy on our citizens, we can spy on theirs, and then we can share the results."

2. Why are so many people angry?

Two reasons: the scope of the spying and its scale. Foreign leaders likely knew the NSA was spying on citizens in their countries, but they are less tolerant of the fact that they were targets themselves.

"What's very important about this particular scandal is that they see our collection as going beyond the pale," Tim Naftali, a senior research fellow at the New America Foundation, tells NPR's Renee Montagne. "We have somehow crossed a boundary that they understood existed and where we've gone they don't accept anymore."

But Boot has a different view.

"Much of their anger is faked for public consumption," he writes. "The only outrage is that anyone is outraged."

3. What are intelligence agencies looking for?

There are two main types of intelligence spy agencies are seeking. The first is the sort that provides information on terrorists, plots and networks, and the work of adversarial governments.

"There was a foiled terrorist plot in 2010 that apparently would have involved attacks in France, the United Kingdom and Germany — so they are aware of the nature of the materials that we are collecting," Naftali says.

Then, there is the sort of information gained from spying on friends like France, Germany and Brazil. Kupchan says:

"One is general information. What's going on in the target country? How is the governing coalition faring? Who's up and who's down? And then, the other [type] would be more targeted information to give the United States a leg up on a particular diplomatic issue: What is the German government thinking about the free-trade negotiation with the United States? What is the German government thinking about sanctions against Iran? These are not questions of direct national security consequence to the United States, but I think intelligence agencies see that kind of information as providing guidance to the diplomats."

4. What's likely to happen next?

Public outrage in Europe — and now anger from allies like German Chancellor Angela Merkel has prompted the NSA to review its surveillance capabilities, "including when it comes to our closest foreign partners and allies."

"This has gone so far and has had such diplomatic repercussions, President Obama cannot help but take it seriously and see it as a reason to alter the way in which we share ... intelligence with Europe. It's going to be very hard," Naftali says, "because, of course, whatever we promise the French and the Germans a lot of other NATO countries are going to want as well. So much of this is going to be done in secret and one thing that we should expect — or should hope for — is that Congress will play a role and we will see a change in the leadership of the NSA — because a signal has to be sent abroad to our allies that we take seriously their concerns about the ambit of NSA collection."

But Kupchan notes there will be "very little" damage even if the U.S. stops spying on its allies.

"And that's because in the end of the day, friends do not mean harm to friends," he says. "The United States would have somewhat less information in its diplomatic quiver, but we could certainly live if there were to be an agreement with our friends to cut this out."

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