Non-relational databases designed for flexible data models, horizontal scaling, and high-performance with unstructured/semi-structured data.
Four main types: Document (MongoDB), Key-Value (Redis), Column-family (Cassandra/HBase), and Graph (Neo4j) databases.
Schemaless design accommodates evolving data structures - ideal for JSON, XML, or dynamic attributes.
Horizontally scalable via sharding, with eventual consistency (CAP theorem tradeoffs).
HBase integrates with Hadoop for random read/write access to HDFS data, while MongoDB is popular for web apps.
Used when relational models are restrictive: IoT data, social graphs, real-time analytics, and content management.