The Dialog search service itself is now owned by a different company, the Thomson Corporation. They are not the same. The difference is how listings are compiled. Search Engines Vs. Search engines, such as HotBot, create their listings automatically.
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. Page titles, body copy and other elements all play a role. 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.