NoSQL (Not Only SQL) databases are a type of database that do not use the traditional SQL language for managing and manipulating data. Instead, they use alternative, often more flexible and scalable data models, such as key-value pairs, document-oriented, or graph-based structures. This makes NoSQL databases well-suited for handling large volumes of structured, semi-structured, and unstructured data, such as the kind that is often found in modern web, mobile, and IoT applications. Some popular examples of NoSQL databases include MongoDB, Cassandra, and Redis.