Solr uses the Lucene Java search library at its core for full-text indexing and search and offers hit highlighting, faceted search, dynamic clustering, database integration, and rich document (e.g., Word, PDF) handling as additional features.
The Islandora Solr search module allows you to search the Solr index. This module includes two blocks that can be assigned different locations. One is a simple search block that searches using the dismax query parser. The other block is called the advanced search block and does fielded searches against the Solr index. Both blocks would use whatever request handler is configured in the module settings.
Chapter 4 - Search and Discovery in Islandora
Chapter 9 - Enabling Indexing & Searching with Solr
Drupal.org modules: