Elasticsearch is a distributed search engine built on Lucene. Documents are JSON. Indexes are like database tables. Shards enable horizontal scaling.
You index documents (write), then query them (read). Queries support full-text search, filters, aggregations, and fuzzy matching.
In system design, mention Elasticsearch when you need search beyond simple database queries. "For product search, I'd use Elasticsearch with an inverted index on product name and description."