NoSQL is still not well understood, as a term or a database market category, by IT decision makers. However, one NoSQL vendor -- 10gen, creators of the open source MongoDB -- appears to be growing into enterprise accounts and distancing itself from competitors. If you're considering, or curious about, NoSQL databases, I recommend you spend some time looking at MongoDB.
Understanding "not only SQL," aka NoSQL
Although the term "NoSQL" suggests a product category that is anti-SQL or anti-relational databases, the term has evolved to mean "not only SQL."
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According to Nosql-database.org, there are more than 122 NoSQL database products to date. These products differ from traditional relational databases in that they don't rely on a relational model, are often schema-free, and support eventually consistent transactions (rather than guaranteed consistent, as in the universally used ACID approach).
Max Schireson, president of 10Gen, asserts that relational databases are here to stay and have an important role to play in tomorrow's enterprise. He sees enterprises using both NoSQL and relational databases, albeit for different applications. (If this positioning sounds familiar, recall that MySQL tried to paint a picture of cohabitation with enterprise database vendors.)
If an application is processing sales orders and needs absolute guaranteed transactions, a relational database supporting ACID transactions is a must. If an application is processing millions of events, such as clickstreams, to better optimize an online sales catalog -- and losing a few of those events is less critical than being able to scale the application and data across commodity servers -- then a NoSQL database could be a perfect fit.