NoSQL (Not Only SQL) is a category of database systems designed for storing and retrieving unstructured, semi-structured, or highly variable data types at scale. Unlike traditional relational databases, NoSQL databases use flexible data models, distributed architectures, and horizontal scalability to handle diverse data formats and high volumes of data. NoSQL databases include document databases, key-value stores, column-family stores, and graph databases, suitable for various use cases such as web applications, big data analytics, and real-time processing.