Use it for the Drupal core module or any module extending its functionality, or aiming to replace it.
The Search module exposes an API other modules can use to search for data in Drupal.
The Node module and the User module integrate with the Search module. The first allows to search in the content of nodes; the second allows to search in the list of users that are registered in a Drupal site knowing the username or (for administrator users) the email address.
Between third-party modules that integrate with the Search module, there are:
- Module that extend the search to new entities
- Modules that allow to use a different search engine, such as the Apache Solr Search Integration module
With search module you can also change the look of search forms with hook_form_alter() Drupal API
