"Backlinks are links pointing at a specific page."
That's the 'official' definition found on Search Engine World. Now the question, and forgive my ignorance, I'm still learning...:
Suppose I did "link:website.com" in Google and G returns a list of 15 results. Now the same command in AlltheWeb returns a list of 390 results.
Could anyone here enlighten me? Why does Google only find a measerly 15 results and AlltheWeb 390? Is one of them wrong, or exaggerating? I don't think AlltheWeb exaggerates, and the reason is that if I search Google just for website.com and then click on the option "Find web pages that contain the term "website.com"" G returns about 400 sites. Examining these sites carefully teaches me that about 80% of these contain not only the term "website.com" but a link as well. Almost all of these pages are relevant to website.com with a few exceptions...
It sort of undermines my belief that Google is the most accurate SE out there...
These days, using "link:website.com" (or "link:www.website.com") in google does not produce very reliable results. Or maybe I should say that google may know about more links than are shown by this command.
Folks are pretty sure that google does not show backlinks that are below a certain PR. But, in addition, there have been some strange changes with the backlinks over the past few months - resulting in a lot of fluctuation in the number of backlinks shown for a site.
This has been discussed a lot recently in this forum. Try searching this forum for something like "link:www.website.com". You might find a better explanation than I've given here. :)
I think it's a safe bet that Google discovers roughly the same amount of links as Alltheweb. As mentioned in another thread, it's guessed that Google obfuscates things to 1. limit reverse engineering and 2. dump all of the non-revenue-generating link: traffic to alltheweb.