Redis is an in-memory data store: it keeps data mainly in RAM instead of on disk, so reads and writes usually take less than a millisecond. It is developed by Redis Ltd. Applications use it as a cache, a session store, a message broker and a database.
Data is stored under keys, and each value has a data type: strings, lists, sets, sorted sets, hashes, JSON documents, streams and others. Commands work on these types directly: INCR adds one to a counter, and a key can expire after a set time. These building blocks make counters, job queues, leaderboards and rate limiters, which cap the requests a client may make in a time window. Redis also offers publish/subscribe messaging, transactions, scripting, and a query engine with full-text and vector search. Client libraries exist for Python, Node.js, Java, Go and other languages.
Redis Open Source is free to download and run under a choice of licenses (AGPLv3, SSPLv1 or RSALv2). Redis Ltd. also sells Redis Cloud, a managed service on AWS, Google Cloud and Azure with a free 30 MB tier and paid plans billed by the hour.