Redis is an open-source, advanced key-value data store, functioning as a data structure server capable of handling over 100,000 operations per second while operating primarily in-memory. It offers persistent storage options and is categorized under NoSQL databases, which are favored for their ability to handle large volumes of unstructured data and their flexibility compared to traditional relational databases. Key features of Redis include simplicity of design, horizontal scaling, and diverse client support for various programming languages.