Lucene is used by many companies and groups as the foundation for their search engines. These organizations include AOL, Disney, and Eclipse. Lucene’s chief selling point is that the indexing engine, with a footprint of a mere megabyte of RAM, can index up to 150GBs per hour of text on commercial off-the-shelf hardware. That’s darn good!

Solr comes into the picture as the search platform front-end for Lucene. It provides full-text search, including the ability to handle such formats as Microsoft Word and PDF with Apache Tika; hit test highlighting; and faceted search, which incorporates free text searching with topic taxonomy indexing.

via Solr: The Most Important Open Source Project You’ve Never Heard Of.

Under the hood, Solr is written in Java and it relies on Lucene for its core functionality.  It usually runs within a servlet container such as the Jetty HTTP server and Javax.servlet.