Search was initially part of the Storage feature and is now a standalone feature.
In eZ Platform, you find 3 search engines possibilities :
Legacy, a Database based search with baseline capabilities
Solrintegration >= 5.4.5
The Search Engines integrations have real search capacities and are not just filtering the results.
eZ Publish Platform 5.4.5 and higher, Solr is the recommended search engine for production use. Solr provides the most most advance search capabilities, and system performance. Currently Elastic Search is experimental, and not professionally supported.