MongoDB (from "humongous") is an open-source document database, and the leading NoSQL database. So the way I think about MongoDB is that if you take MySql, and change the data model from relational to document based, you get a lot of great features: embedded docs for speed, manageability, agile development with schema-less databases, easier horizontal scalability because joins aren’t as important.
If you’ve built an application that stores its data in a relational database like MySQL or PostgreSQL, then you’ve likely run into situations where joining two or more large tables becomes very slow and painful. Because our SQL Layer stores its underlying data in our Key-Value Store, it has a unique feature called “Table Groups” that alleviates painful joins and allows your application to stay speedy.
NoSQL. Next Generation Databases mostly addressing some of the points: being non-relational, distributed, open-source and horizontally scalable. Wide Column Store: Hadoop / HBase, Cassandra, Hypertable, Document Store: MongoDB, Elasticsearch