Redis is an open source, advanced key-value store. It is often referred to as a data structure server since keys can contain strings, hashes, lists, sets and sorted sets.
You can run atomic operations on these types, like appending to a string; incrementing the value in a hash; pushing to a list; computing set intersection, union and difference; or getting the member with highest ranking in a sorted set.
In order to achieve its outstanding performance, Redis works with an in-memory dataset. Depending on your use case, you can persist it either by dumping the dataset to disk every once in a while, or by appending each command to a log.
Redis also supports trivial-to-setup master-slave replication, with very fast non-blocking first synchronization, auto-reconnection on net split and so forth.
Other features include a simple check-and-set mechanism, pub/sub and configuration settings to make Redis behave like a cache.
You can use Redis from most programming languages out there.
Redis is written in ANSI C and works in most POSIX systems like Linux, *BSD, OS X and Solaris without external dependencies. There is no official support for Windows builds, although you may have some options.