NoSQL is a type of database designed to handle large volumes of data that don't fit neatly into traditional tables. Unlike SQL databases, which use structured query language and fixed schemas, NoSQL databases are more flexible. They can store data in various formats, such as documents, key-value pairs, or graphs, making them ideal for applications that require quick access to diverse data types. These databases are particularly useful for big data and real-time web applications, where speed and scalability are crucial. Popular NoSQL databases include MongoDB, Cassandra, and Redis, each offering unique features to meet different data storage needs.