You know that Google uses back links to size up your website and determine its ranking. You also know that it wants to see quality back links, and if you want to rank high on the search engine results pages, you need to get quality links that are relevant to your keywords. But what is a quality link? If you're not sure how to decide whether you should try to get a back link from a particular page or web site, this article may help guide your decision.
Link building is the most important part of search engine optimization activity targeting the Google search engine. As an overview, this search engine relies on links to determine the reputation of the page. This is analogous to citations in the world of scientific or academic papers. A single quality and relevant link from a trusted and authoritative source is equivalent to thousands of low-quality and irrelevant links.
It is common sense that links from these types of sources are desirable because they are powerful. Sources of such links speak with authority and are popular and expert in their own fields. Therefore if you get references from these types of documents, there is something in your work that catches their attention, and in the long run, you may become trusted and authoritative also.
For example, in the scientific or academic environment, scientists and scholars often cite each other in research papers, to give reference or acknowledge a certain source. Of course, the more trusted and authoritative the source, the higher the number of citations it receives from other published scientific or academic works.
One example of this is Albert Einstein's paper on the Theory of Relativity, where Einstein cited Isaac Newton's Universal Laws of Gravitation. The same concept works when applied to the online world. Quality documents are referenced by other quality and relevant documents.
Larry Page and Sergey Brin, researchers from Stanford University, translated this very simple analogy of finding quality documents in the academic world into an application in the online world, resulting in the birth of Google and its algorithm: Page rank. Google then leads the multi-billion dollar search industry that still remains strong even today (2009).
This article aims to educate the search engine optimization community as to what particular qualities or factors the search engines used to rate the quality of the linking page in assessing back links.
Link Trading 
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