The National Security Administration is the eyes and ears of American national defense, charged with maintaining vigilance against external threats through information gathering — be it eavesdropping phone calls (like the one that led to bin Ladin's downfall), surveillance video or photography.
Once collected the data is parsed, keywords are all flagged and organized for immediate action. Or at least that's how it's supposed to work.
Instead, the NSA often collects so much data that it simply cannot analyze it all. In fact, every six hours, the NSA pulls in an estimated 74 terabytes worth of raw data. That's roughly the same amount as is stored in the The Library of Congress' digital archives.