Lucene is the "internals of a search engine", released by the Apache Software Foundation. It contains matching models etc, but not user interface and no collection retrival (ie a crawler).
Lucene's indexes are described here.
Homepage: ![[WWW]](/wikis/modern/img/moin-www.png){kind=small}
http://jakarta.apache.org/lucene/ Wiki: http://wiki.apache.org/jakarta-lucene