Although the main fundamental backing for all of the major search engines is the same, there are some essential differences. The differences may be small, but they lead to large-scale changes on SERPs. The various algorithms amongst the different engines are widely opposite and ever changing.
Yahoo! and MSN consider on-page keyword factors to be of primary importance, whereas Google chiefly relies on link building. Also, sites that have been indexed by Google for a long period of time gain substantial credibility, whereas Yahoo! seems to render no preference. Therefore, a Google PageRank may take substantially longer than that of a Yahoo or MSN page ranking.
Tomorrow, I will go into further detail and examine each search engine more thoroughly.