NoSQL databases are a category of database management systems designed to handle large volumes of unstructured or semi-structured data, which traditional relational databases may struggle with. Unlike their relational counterparts, NoSQL databases offer flexible schema designs and can scale horizontally across distributed systems. They are particularly well-suited for big data applications, where data variety and velocity are high. Key types of NoSQL databases include document stores, key-value stores, column-family stores, and graph databases, each optimized for specific types of data and access patterns. For example, document stores like MongoDB are excellent for hierarchical data structures, while key-value stores such as Redis are ideal for fast, in-memory data retrieval. By accommodating diverse data models and large-scale data management, NoSQL databases support dynamic and complex applications, making them a critical component in the big data ecosystem.