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Riak combines a decentralized key-value store, a flexible map/reduce engine, and a friendly HTTP/JSON query interface to provide a database ideally suited for Web applications.

A document-oriented Web database

Schema-free document databases are in many ways a more natural fit for Web applications than the traditional RDBMS. With interfaces that are natural and convenient for rapid development, and a lack of object-relational mappers and other heavy middleware, applications can be both simpler and more powerful.

A database at Web scale

Developed with solid distributed systems fundamentals from the start, Riak scales out to meet your needs. It also scales down easily, making development and prototyping easy.

A database that eases Web operations

A truly fault-tolerant system, no aspect of Riak involves single points of failure. Since no machines are special or central, the user of Riak can decide exactly how fault-tolerant they would like their application to be.

you can find more here

and a perfect howto to install it here