https://www.schneier.com/blog/archives/2014/03/metadata_surv...

Quote:

"An easy thought experiment demonstrates this. Imagine that you hired a private detective to eavesdrop on a subject. That detective would plant a bug in that subject's home, office, and car. He would eavesdrop on his computer. He would listen in on that subject's conversations, both face to face and remotely, and you would get a report on what was said in those conversations. Now imagine that you asked that same private detective to put a subject under constant surveillance. You would get a different report, one that included things like where he went, what he did, who he spoke to -- and for how long -- who he wrote to, what he read, and what he purchased. This is all metadata, data we know the NSA is collecting. So when the president says that it's only metadata, what you should really hear is that we're all under constant and ubiquitous surveillance."