Elasticsearch is a text search software created by Shay Banon that uses Lucene for its text search capabilities. It has a RESTful API and supports features like aggregations, scaling clusters, and sharding for performance. Documents are stored in indexes which contain types that define the fields for documents. Queries can be used to search for documents, including leaf queries that search single fields and compound queries that combine criteria. Advanced topics include joins, geospatial queries, aggregations, and plugins.