What exactly is a backlink in Google's eyes?

[dimmer]

Does anybody know what the exact criteria of a backlink in goole is?

I can search for a domain and see hundereds of pages in the serps, from hundreds of sites. I can check the link command and only see a handfull of backlinks.

Does a link to your site have to be in html?
Do non html links count?

Does the link have to have a certain PR?

[g1smd]

Google shows only a small sample of links when you use the link: operator.

Yahoo shows many more.

[tedster]

Do non html links count?

Not usually for the transfer of PR, to my knowledge. Non-html may be used to discover new URLs if the text of the link is easily machine-readable from the flash, or javascript, or whatever.

Links in .doc and .pdf documents are still an open question for me. Most of the time there's so little PR there to transfer anyway.