The National Security Agency headquarters in Fort Meade, Md. (NSA via Getty Images)

A leaked court document appears to show that the National Security Agency, a U.S. intelligence agency that specializes in data collection, secretly ordered Verizon to hand over customer information for tens of millions of Americans. The order was first reported by The Guardian's Glenn Greenwald; also see this report by the Washington Post's Ellen Nakashima.

The court order asked for "telephony metadata" about Verizon customers; it is likely part of an ongoing data request that stretches back to 2006. Here's what the NSA could likely learn about you, if you are a Verizon customer, from the data it requested:

• What phone numbers you called, when and for how long

• Any calling card numbers you used

If you're calling from a cell phone, a special 15-digit International Mobile Subscriber Identity code that law enforcement sometimes uses to derive a cellphone's physical location

Law enforcement can also uses the data en masse, collecting it into a giant database that would allow it to search for and identify trends, patterns or things it might want to investigate further.

Here are the things that law enforcement can't learn from the requested date:

• The address associated with the phone number

• The actual content of the calls

That last point is important: These are not wiretaps. This document leak does not mean that the NSA is listening to all of these phone calls.

But it does mean that the U.S. government can tell if, for example, you've been making short calls to the Pakistani tribal regions every other week for the last five years. Or if you placed a call to a region of Yemen associated with terrorist groups shortly after one of those groups launched a new plot. Or if you used a calling card that was previously used to dial a Chinese military facility. Or if you're a mid-level NSA official, maybe concerned about your agency's broad snooping, who placed calls to a cellphone owned by a newspaper reporter.