android - What protocol does a headphone jack use?

I have 2 questions. One is simple, the other may be not.

1. What's the official name for the standard headphone jack? It's universal for all brands of phones, so it must have a name (when you want to go shopping for components).

2. Is it using some sort of a communication protocol? I know for a fact that it's not only for headphones. There is a device for scanning credit cards that plugs into the same jack. I believe Nexus also has a TV-out support from the same jack. Basically, can it be used for other I/O applications?

Thank you