What are the types of NoSQL databases?

There are four main types of NoSQL databases:

1. Key-Value Stores: Store data as key-value pairs. Simple, scalable, ideal for caching, session data, configurations. Examples: Redis, Memcached, Amazon DynamoDB (often used as KV), Riak.

2. Document Databases: Store data as documents (usually JSON, BSON, XML). Flexible schema, suitable for storing complex, nested data. Good for content management systems, product catalogs, user profiles. Examples: MongoDB, Couchbase, OrientDB (hybrid).

3. Column-Family Stores: Organize data into column families. Optimized for writing large volumes of data and reading by row or column ranges. Suitable for time series, logs, analytics. Examples: Apache Cassandra, HBase, ScyllaDB.

4. Graph Databases: Store data as nodes and relationships (edges) between them. Optimized for traversing relationships. Used in social networks, recommendation systems, fraud detection, network topologies. Examples: Neo4j, ArangoDB (multi-model), Amazon Neptune.