The term search engine is often used to describe both true search engines and directories. They are not the same. The difference is how listings are compiled.
Search engines crawl the web, then people search through what they have found.
If you change your web pages, search engines eventually find these changes, and that can affect how you are listed. A directory such as Yahoo depends on humans for its listings. You submit a short description to the directory for your entire site, or editors write one for sites they review. A search looks for matches only in the descriptions submitted. Changing web pages has no effect on your listing.