External Links
External links, or backlinks, are any link that points to your site from a different site. One of the more important ways Google,Yahoo, and MSN search engines decide which sites to display in the search results and where to rank them is by the number and quality of links that point to the site. You can get a sense of how many external links any site has by typing link:www.domain.com into the search box of Google or Yahoo. For instance, I just typed link:www.google.com in there and see that Google has 538,000 external links. They have a lot of links and that is because they are a very popular site.
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Try the same thing in Yahoo, and you'll see a different number. If fact, Yahoo shows over 5,000,000 links pointing to Google. The reason Google & Yahoo report different numbers is that Yahoo is showing every link they can see that points to your site, but Google is only showing those links that match the Google criteria of "important link." They do see all the links, though, and they count the total number of links coming into your site as a positive, even if they don't report them all.
Most search engines use external links as a measure of how popular a website is, and popularity, they reason, is good indication of how useful the website's content is. All backlinks are good, but some are better than others.
All that being said, Google rates links in a couple of different ways. One is a quality score they call "PageRank" that is assigned to every page in their index. The simplified version of this score is expressed as 1/10 or 4/10 or whatever with 10 being the highest. Google.com has a 10/10 of course. Now, they will let us see an approximation of the score, but they won't let us see the actual score. If you have the Google toolbar installed in your browser, you can choose to have a PageRank meter show for every page you visit.
The PageRank score is mostly based on the number of web pages that link to you, the relative quality of those pages, and the relevance of those pages. So, if you have 10 links pointing to your homepage, and 9 of them have a 1/10 PR score and the 10th has a 5/10 score, you would have a probable PR of the average of that, 1.4/10. There is also a weighted value given for links that appear to Google to be topically related to yours. In theory, if you have 10 links all at 1/10 but 5 of them are from other sites that are similar in theme to yours, those 5 would count higher. They round the number, of course, and also factor in a host of different variables that can only be described as voodoo. It's all very complex. Distilled down, we'll just say that:
- Any link to your site is good.
- Links from important sites are better.
- Links from important sites that are thematically similar to yours are best of all.
Just to make things even more interesting, search engines do all this calculating on the fly for every different search query, using the actual search term used as the basis for relevancy. So, suppose your cowboy boot merchandizing site has 65 million external links pointing to it, but every single one of them looks like this:
Do you suppose you'll rank higher for the search term "Cowboy boots" or for the search term "Click here"? If you said "Click here" you win a prize. The engine uses link text to assign meaning to the links they encounter. (If you're not familiar with the term, link text is simply the part of a link that is underlined.) So, now, you not only have to worry about how many backlinks you have, but you also have to worry about the link text pointing to your site.