Apache Solr is a highly scalable, open source search platform built on Apache Lucene. It provides full-text search, faceted search, real-time indexing, dynamic clustering, database integration, and rich document handling, making it a popular choice for powering search on large content repositories and e-commerce sites.
Solr exposes a REST-like API over HTTP and supports JSON, XML, and CSV for both indexing and querying. It can run standalone or in distributed mode (SolrCloud) for high availability and horizontal scalability.