Apache Solr vs Lucene: What are the differences?

Apache Solr: An open source search platform. It uses the tools you use to make application building a snap. It is built on the battle-tested Apache Zookeeper, it makes it easy to scale up and down; Lucene: A high-performance, full-featured text search engine library written entirely in Java. Lucene Core, our flagship sub-project, provides Java-based indexing and search technology, as well as spellchecking, hit highlighting and advanced analysis/tokenization capabilities.

Apache Solr and Lucene can be categorized as "Search Engines" tools.