This document provides an overview of Elasticsearch, including: - It is a NoSQL database that indexes and searches JSON documents in real-time. Documents are distributed across a cluster of servers for high performance and availability. - Elasticsearch uses Lucene under the hood for indexing and search. It is part of the ELK (Elasticsearch, Logstash, Kibana) stack and is open source. - Documents are organized into indexes and types, similar to databases and tables. Documents can be created, updated, and deleted via a RESTful API.