What are the main categories of non-relational database management systems?

Non-relational database management systems (NoSQL) are usually divided into several main categories:

• Document-oriented: store data in document formats (e.g., JSON, BSON). Example — MongoDB.
• Key-value: simple key-value pair storage structure. Examples — Redis, Riak.
• Column-oriented: data is stored by columns, which is efficient for analytics. Examples — Apache Cassandra, HBase.
• Graph databases: focused on storing and processing graphs (nodes and edges). Example — Neo4j.

Each type is suitable for different tasks depending on the data structure and performance requirements.