PRISM, Blarney And Metadata

The big noise right now is the revelation concerning the US government's PRISM and Blarney surveillance programs gathering data on phone calls, emails and so on. Much is being made about the claim that the process does not "listen in" to phone calls, but rather collects and disseminates metadata. Assuming that is true (and these days, who really knows), what is metadata? In effect, it is data about data. Even if the actual audio content of a phone conversation is not recorded, the metadata describes information such as the parties involved in the call, the time the call was made, the length of the call and possibly the physical location if GPS or cell tower data is available. You can see that this could be quite sensitive, embarrassing or damning - and this is being done to US citizens in a rather sweeping manner.