Search engine optimization techniques for Google and Yahoo are quite different. Many websites rank well in one search engine but not the other. This is the direct result of each search engine having its own unique ranking algorithm. For example, the Google algorithm predominantly values the anchor text of in-bound links. Yahoo places more emphasis on keyword density and meta tags.
The primary reason for the difference in ranking algorithms is that Google owns the patent on Page Rank (PR), named after Google's founder Larry Page. As a result of owning this patent, other search engines need to place more emphasis on different optimization factors including website URL, keyword density and so on.