In short: cables are the official record and communication tool of the diplomatic corps. As several other replies have pointed out, cables are a form of electro

In short: cables are the official record and communication tool of the diplomatic corps. As several other replies have pointed out, cables are a form of electronic communication, but they aren't emails. That may sound like a meaningless distinction, but in the everyday business of the department it is real. If you work at DOS, you usually have two email addresses--one on the unclassified and one on the classified network. Additionally, you can have access to a system to read, review and send cables. Think of them as official inter-office memos, or something like that. They're formatted in a specific way, and they do not, under any circumstances, travel on any communications systems not completed operated by the US Government. You get the sense of what I mean by format if you look at some of the documents on the NYT site (or sit down, as I have, and read a few thousand cables in a row). If you're the Ambassador to Country X, and you have a meeting with the President of X, you write a memo, in a particular format, with each paragraph classified U/SBU/C/S/TS, and in a pretty particular tone. That memo (cable) then gets sent to, who knows, the Assistant Secretary for the Region X is in, an Undersecretary or two, the OPs center, the Deputy, etc.