International Article Numbers).
I don't know how widely they're used, and they're generally not as consumer-visible as ISBNs for books. The iPad I recently bought has a UPC barcode on the box, but not on the machine itself, and I've never had occasion to use a UPC or EAN to look something up.
This is a thread where people can ask questions that they would ordinarily feel embarrassed for not knowing the answer to. The previous "stupid" questions thread is at almost 500 questions in about a month, so I think it's time for a new one.
Also, I have a new "stupid" question.