What is Redis, and how is it different from a traditional relational database?

Redis is an in-memory data structure store that can function as a database, cache, message broker, and streaming engine. Unlike RDBMS (rows, tables, SQL, disk-first), Redis stores data in RAM using native data structures (strings, lists, sets, hashes, sorted sets), giving sub-millisecond latency. It has optional persistence but is not designed for complex relational queries or full ACID transactions across multiple keys.