Redis is an open-source, in-memory data structure store used for caching, session storage, queues, and analytics. It supports common data structures like strings, hashes, and sets. Redis commands operate on these data structures, providing features like keys with time-to-live and user-defined data structures through modules. Redis 5.x introduced modules to extend Redis capabilities with new data structures in a low-latency manner. The document discusses using Redis from Java applications through libraries like Lettuce and Spring Data, and provides exercises to practice Redis commands and build services using Redis data structures.