Relevancy of links
Link Relevance is a function of the subject matter of both the site that provides the link and the site that is linked to.
For instance, a web page that is regarded by Google as being about "art and design" information, that links to another web page that features content about "art and design", will pass on Link Relevancy to the page being linked to.
This is better for the site being linked to than simply receiving a link from a site about an unrelated subject.
i.e. Site A is about "art and design". Site A links to Site B. Site B features content related to "art and design". Therefore, Site A is "voting" for Site B also being about "art and design". Given the fact that Site A is relevant to the subject "art and design", Site B is given an additional "boost" due to Link Relevancy.
The ultimate type of inbound link, then, is obviously a relevant link from an "authority" site (see the Authority and Credibility section).
It is impossible to be accurate in calculating the value of such a link, but it is certain that a single link from a relevant, "authority" site is worth many links from non-relevant or "non-authority" sites.