NoSQL databases are a category of non-relational databases that provide an alternative to traditional SQL databases. They are designed to handle a variety of data types, including structured, semi-structured, and unstructured data. Unlike SQL databases, which rely on a fixed schema and structured query language, NoSQL databases offer more flexibility in terms of data storage and retrieval.

There are four main types of NoSQL databases: key-value stores, document stores, column-family stores, and graph databases. Key-value stores, for example, store data in pairs of keys and values, similar to a dictionary. Document stores organize data in documents, often using JSON or similar formats, allowing for complex data structures. Column-family stores group data into columns rather than rows, while graph databases focus on relationships between data points.

NoSQL databases are particularly useful for applications that require scalability, rapid development, and the ability to handle changing data structures with minimal downtime ³⁵.