When a site grows and content gets edited or deleted or when content architecture changes it is very often that the site is left with URLs that do not point to valid pages/content/menus in Drupal. Typically these URLs would result in a 404 error and ends up showing the default "404 Page not found" page in Drupal. However the Search 404 module allows the site owner to direct these users to relevant content in the site based on the keywords in the URL that resulted in the 404 error.
The Search404 module does not offer any search capability on its own. Instead it uses one of the search modules that is installed in the site. The module supports integration with Apache Solr, Lucene, Xapian, Fuzzy Search and Search by Page - in addition to the Core Drupal Search. So one of these have to be present and enabled on the site for the Search404 to work.
The module also offers some SEO capabilities when used with Core Search, Apache Solr, Lucene, Xapian where it can redirect visitors to the first result as a 301 or 302 redirect instead of the 404 error. This could help sites retain the pages in the search engine index.
When the 404 error is resulted from a result in a search engine (one of Google, Yahoo, Altavista, Lycos, Bing and AOL) then the module could also retrieve the original keywords used in the search engine instead of the keywords in the URL.
There is also a regular expression filter to filter out keywords that you do not want to use in the search when the 404 error occurs.
The module also allows the administrator to customize the message that is shown to the user when the results are shown after a 404 search.