NoSQL is a broad category of databases that move away from the relational model (tables + SQL). It covers very different systems: document, key-value, columnar, and graph. Not "better than SQL" — simply suited to a different class of problems.
Main types
Document (MongoDB, CouchDB) — store JSON-like documents. Flexible schema, convenient for nested data
Key-value (Redis, DynamoDB) — ultra-fast access by key
Columnar (Cassandra, ClickHouse) — efficient for large-scale analytics
Graph (Neo4j) — for complex relationships between entities (social networks, recommendations)