Redis is an opensource data structure store, it is distributed under BSD license. Redis stands for Remote DIctionary Server. Redis is extremely fast Inmemory data store. It does not support SQL and that makes it much faster. Redis store the data in Key value pair. Accessing the data based on key. Data can be any binary data. Redis support different data structure like abstract data structures, such as strings, lists, maps, sets, sorted sets, hyperloglogs, bitmaps, streams and spatial indexes. Along with inmemory data structure, it also support storing the data on disk for persistency, with little compromise on speed. Almost all languages provide the interface towards redis for storing and accessing data.
The database is free and open source with a commercial license available (MySQL is now owned by Oracle after they bought Sun). Common applications for MySQL include php and java based web applications that require a DB storage backend, e.g. Dokuwiki, Joomla, xwiki etc
MySQL has one major advantage, since it is free, it is usually available on shared hosting packages and can be easily set up in a Linux, Unix or Windows environment. If a web application requires more than database, requires load balancing or sharding