Ask HN: How is automatic video tagging is done based on its content?

If you're referring to the way, e.g. Youtube, automatically detects the author of the video or artist upon upload, then this article will give you an overview about tagging "audio":

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

One way might be to run the audio stream through a speech-to-text engine and parse the resulting transcript.

A video recognition system could also be used to identify faces, landmarks and common objects.

Can you give an example of a website using automatic video tagging first?