This document provides an overview of NoSQL databases, including:
1) NoSQL databases differ from relational databases by being schemaless and not using joins. They were created to address limitations of relational databases.
2) NoSQL databases can be categorized as key-value, column-oriented, document-based or graph-based. MongoDB is presented as an example document-based NoSQL database.
3) The CAP theorem states that a distributed system cannot simultaneously provide consistency, availability, and partition tolerance. Pros and cons of NoSQL databases are discussed.