There appear to become a large amount of new "NoSQL" type databases available. A few of the popular ones are CouchDB, Cassandra and MongoDB.

Do you know the variations between such databases and just how could they be not the same as tradition relational databases? What exactly are the pros and cons of picking NoSQL DB's over SQL DB's?

The word NoSQL covers lot of different methods to data storage varying in the easiest key/value storage to classy document databases. It is a appealing buzz word, but not so discriptive IMHO.

For any quick intro you can have a look at the Wikipedia entry for NoSQL

They aren't better. NOSQL does not involve any new innovation or special feature. NOSQL just describes an accumulation of software items which are used for several kinds of application try not to always cash else that is similar to one another. NOSQL doesn't have to mean a non-relational database.

Agreed, now you ask , "not what's best,Inch it's "which solution or group of solutions is the best for this specific situation."

NoSQL covers lot of different storage technologies for example CouchDB, MongoDB, Cassandra and Solr.

CouchDB and MongoDB store multi-dimensional data-structures. MongoDB can also be schema-less. Cassandra is really a column-based storage engine for fast retrieval, and Solr helps solve other issues for example faceting.

NoSQL simply describes any storage facility which isn't interacted with via SQL queries.