The National Security Agency says it doesn't target American citizens when searching through its vast quantities of data looking for threats, but that doesn't mean it won't look through your email. The New York Times has the latest revelation in the never-ending flood of leaks about how NSA conducts its business, and the newest one suggests that thousands and thousands of emails, including those sent and received by American citizens, are being combed over by intelligence agents without a warrant.
The description of the program is a little confusing, but (we think) it works like this: Say NSA the wants to find out information about a particular individual or "target." Since NSA is already authorized to intercept any electronic communication that crosses U.S. borders, their servers are already soaking up this data on an ongoing basis. (And that probably doesn't even include the "massive amounts" of data that countries like Germany give to the NSA everyday.)
To find out more about the target, NSA copies a large selection of the data and runs a keyword search for something very specific, like a name or an email address or a phone number. In "a small number of seconds," the program then take any communication that matches that search and sets it aside for actual humans to look at later. The rest is deleted.