Elasticsearch is a JSON document database that allows for powerful full-text search capabilities. It uses Lucene under the hood for indexing and search. Documents are stored in indexes and types which are analogous to tables in a relational database. Documents can be created, read, updated, and deleted via a RESTful API. Searches can be performed across multiple indexes and types. Elasticsearch offers advanced search features like facets, highlighting, and custom analyzers. Mappings allow for customization of how documents are indexed. Shards and replicas improve performance and availability. Multi-tenancy can be achieved through separate indexes or filters.