Lucene is the core search engine, it can be used on its own and has a lot of depth in its many capabilities. ElasticSearch (and SolrCloud >> LucidWorks Fusion

Lucene is the core search engine, it can be used on its own and has a lot of depth in its many capabilities. ElasticSearch (and SolrCloud >> LucidWorks Fusion Server) add a distributed architecture that leverages Lucene's capabilities. LogStash helps ES easily deal with log files, and it has been one of the main marketing drivers used by elastic which is ironic since most of Lucene's capabilities are more useful on human generated text as opposed to machine generated logs.