Drupal uses search indexing using content type, classification information from taxonomy system and the content meta data.
Drupal treats its content as fine-grained semantic information that Drupal knows about can be used to fine-tune search results.
Also, you can tweak and override the search system to adjust the user interface, the way content is ranked, and the way results are displayed. That said, Drupal integrates well with external search engines such as Apache Solr, Xapian, and Sphinx if the built-in search does not meet your needs very easily.