Very. Search engines Google, Yahoo!, MSN, and Ask Jeeves all consider the amount and more important the quality of back links (i.e., inbound page links) perhaps the single most important factor in their ranking calculations for indexed Web content. In essence, the search engines consider the presence of an inbound link to a given Web page an indication that the page from which the link originates considers the page it is pointing to important enough to actually link to. Additionally, the anchor text of the back link will help the search engine understand why the page is important. A search engine's link quality analysis further examines the relative importance of the page that contains the outbound link. In short If, for example, a page that holds a Google PageRank of 8 points to a given Web page, then theoretically that link will have a more positive influence on the linked-to page's search engine ranking than it would had the link originated from a page with a PageRank of 3. This, of course, assumes that the search engines trust that the back link is bona fide and not part of a link buying/link farming scheme.
The extreme importance of links to a Web page (and, to a lesser extent, from a Web page) means that link building must be an integral element of any successful search engine optimization campaign. It is critical to understand that search engines will carefully examine the origin and anchor text of a page's back links before determining how those links will influence the page's search engine ranking. This means that a handful of quality links generally will have a bigger impact than a large number of less-important links.
Lately, the top search engines, notably Google and Yahoo!, have employed so-called sandboxing measures to ensure that no new Web page's ranking is allowed to skyrocket until its links and content have been examined carefully. The sandboxing period, generally a few months, applies to most newly-indexed pages and allows the search engines to ascertain that a page's back links do not originated from a link farm or other form of search engine spamming. The sandboxing concept can be frustrating for Web site owners, but it further increases the importance of link quality in relation to search engine ranking.
