Chapter 26 GLOSSARY OF TERMS and ACRONYMS:

Cable/Telegram Used interchangeably to refer to communications (messages) sent electronically.

http://www.state.gov/documents/organization/22249.pdf

I'm under the impression that a cable is something of a formalized document format - like when an enterprise has a formal template for a memo. Link wise, I'm pretty sure embassies use a combination of leased lines and satellites, with link layer hardware encryption and one time pads for messages deemed sufficiently sensitive. They may well get delivered using smtp but every email wouldn't get called a cable, and definitely not over public networks. The name cable obviously just stuck from historical use.

Dates back to when messages were transmitted by telegraph - transmission via cable was for really urgent stuff that couldn't wait for the diplomatic bag.

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.

even more OT, but an interesting story about decryption and leaking of cables is the Zimmerman telegram - the cable that was decrypted and leaked and caused the USA to enter the first world war - from a time when cables literaly went over the cable. This is from the days before computers, when intelligence and human decryption were used.

http://en.wikipedia.org/wiki/Zimmermann_Telegram

So the decryption and leaking of cables is nothing new, but this is a whole new scale. Sometimes I wonder if Wikileaks is now doing the bidding of governments without realising it. It's unlikely now but maybe in the future will be so.

I'm assuming it's just the general term for a 'piece of transmitted information', no matter what the medium.

As I understood by reading the articles, they have their own "Internet" through which they communicate via cables, and every cable has it's own strict structure.