Each search service has its own characteristics. Knowing those characteristics will help you design your site to be ranked higher by the services. Most search services provide Help information that describes how you can improve your ranking. The following links take you to the Help information.
Keep in mind, however, that search engines and directories are not static. They are continually being updated with more refined search algorithms to give better service and to defeat people who try to abuse the engines and directories to get higher rankings.
A strong trend in search services is the returning of paid advertisements as part or all of the search results. Click here for more information about this.