External Factors Impacting SEO
The major external factors that impact the search engine optimization of your pages are the inbound links from other sites. The influence of this links is dependent upon three variables of the links themselves.
Number of Links
There is no magic number of links that should be directed toward your page from other sites. All other things being equal, the more links you have the higher you page will climb in the search engine listings for a particular keyword or key phrase. However, of course, «all other things» are rarely equal.
Anchor Text of the Links
What is also important with the external links in addition to how many is what the anchor text (or linking text) is. Just as with your internal links, the wording of the exact link is taken by the search algorithm as a description of the page content of the page to which that link is directed.
You are not always able to control the anchor text of external links. Sometimes the referring webmaster will link the words, «click here» or «this site» or even your URL. Never turn down such links.
However, if you have some influence over the anchor text, try to have the anchor text be very similar to your page’s title or the links that you place on your own site that point to the page. That provides additional data, a sort of confirmation that the page is really about what you claim it is.
Reputation of the Linking Site
A link that comes from a site that a search engine considers an authority is better than a link from a run of the mill site. Authority sites are those that have a lot of inbound links themselves or that the search engine’s human reviewers have declared to be important.
Just to give you a feeling for what an authority site is, I’ll give a very few examples. The DMOZ and Yahoo directories are authority sites. The New York Times is an authority site. Of course there are many more, most of which are relatively unknown to the general public. Some are viewed as authorities within their own niches but not for other niches.