Apache Solr is a very popular open source search platform, based on the Java Lucene Library. Solr is very stable, scalable and reliable and provides a wide set of core search functions. Solr creates an index of the available documents and then you can query Solr to return the most relevant ones for your search.
This tutorial will deal with the integration between Drupal and the Solr platform. Before you begin, you will need to have installed Apache Solr on your server.