Search Engines use proprietary algorithms–normally regarded as “highly secret”–to calculate a number of components of your overall ranking score. Though the algorithms vary from search engine to search engine, it is commonly known that all of the following are taken into account: content (and the optimization thereof), images, links (both internal and external), page titles, META descriptions and keywords*, URL and longevity, and overall traffic from within that search engine.
*It is arguable that search engines no longer use information provided in the “META Keywords” section of the META tags. Instead, they calculate their own keyword ranking percentages based on the page content.