Elasticsearch is a distributed, open source search and analytics engine based on the Apache Lucene library. It allows for schemaless documents stored in JSON format to be indexed and searched in near real-time. Elasticsearch uses concepts like clusters, nodes, shards, and replicas to provide scalability. It exposes various APIs to perform operations on documents, aggregations, search, and manage indices and clusters. Mappings define how documents are stored and indexed, while analysis converts text to tokens using tokenizers and filters.