Imagine you hired a detective to eavesdrop on someone. He might plant a bug in their office. He might tap their phone. He might open their mail. The result would be the details of that person’s communications. That’s the “data.”
Now imagine you hired that same detective to surveil that person. The result would be details of what he did: where he went, who he talked to, what he looked at, what he purchased — how he spent his day. That’s all metadata.
This is not to say that what the NSA does is right or wrong, but this does set forth a clear definition of what actually is the content under contention. The above is the bulk of the article, the analogy is really spot on as what consists of metadata, but the rest is worth reading since the implications of this categorization are made inarguably clear. For those who wish to dive deeper, Mr. Schneier linked to a lengthy but fascinating article on how metadata can actually be used.