NoSQL databases embarks a new stream of data storage in the form of Key value pair or documents. There are a number of open source NoSQL database such as MongoDB, CouchDB, Apache Cassandra to name a few. These databases are distributed in nature and provides much higher performance compared to relational databases. but they loosely adhere to the ACID properties of a relational database. It is designed to handle very large amounts of data spread out across many commodity servers while providing a highly available service with no single point of failure.
MongoDB is designed for problems that aren't easily solved by traditional RDBMSs, including problems that require databases to span many servers. MongoDB is a document-oriented database. This means that unlike a relational database management system, MongoDB manages collections of JSON-like documents.
Cassandra provides a structured key-value store with eventual consistency. Keys map to multiple values, which are grouped into column families.