NoSQL DB, as name suggests there is no SQL language support. No SQL mean no join and no relation.
NoSQL db are non RDBMS database system used with high data intensive web application. But these DBMS don?t support ACID. These db are typically key value pair store, where you can store object?s after serializing them.
NoSQL DB can be categorize into following
- Key-value store
- Document store
- Hierarchical key-value store
- Hosted services
- Key-value cache in RAM
- Key-value stores on solid state or rotating disk
- Ordered key-value stores
- Multivalued databases
- Object database
There are hundreds of implementation of these type of NoSQL dbs.
There are some famous NoSQL dbs like Mango DB, Apache Hadoop etc.
You can also implement your own.