The Search module is a core module that exposes an API other modules can use to allow searching for data in Drupal.
The Search module exposes an API that other modules can use to allow searching for data in Drupal. The Node module, and the User module implement the integration with the Search module, allowing searching of the content of nodes, or the list of users that are registered in a Drupal site knowing the username, or (for administrator users) the email address.
Third-party modules can integrate with the Search module, which allows the users to look for different entity. Some of them implement the integration with the Search module to allow Drupal to use different search engines; one of those modules is the Apache Solr Search Integration module.