bigtable vs cassandra vs simpledb vs dynamo vs couchdb vs hypertable vs riak vs hbase, what do they have in common?

Sorry if this question is somewhat subjective. I am new to 'could store', 'distributed store' or some concepts like this. I really wonder what do they have in common and开发者_StackOverflow want to get an overview on all of them. What do I need to prepare if I want to write a product similar to this?

The NoSQL Database site summarizes the concept like this:

Next Generation Databases mostly address some of the points: being non-relational, distributed, open-source and horizontal scalable. The original intention has been modern web-scale databases. The movement began early 2009 and is growing rapidly. Often more characteristics apply as: schema-free, replication support, easy API, eventually consistency, and more. So the misleading term "nosql" (the community now translates it mostly with "not only sql") should be seen as an alias to something like the definition above.

That site also maintains an archive of articles on NoSQL databases. Most of them seem to focus on particular products but there are some more general overviews. If you are serious about building your own one then Design Patterns for Distributed Non-Relational Databases does a good round up of things you need to consider.

Good overview of the nosql world: http://www.vineetgupta.com/2010/01/nosql-databases-part-1-landscape/