Cassandra - why?

http://www.datastax.com/wp-content/uploads/2011/02/DataStax-cBackgrounder.pdf

Cassandra Client Requests

http://www.datastax.com/docs/1.0/cluster_architecture/about_client_requests

Cassandra Snitches

http://www.datastax.com/docs/1.0/cluster_architecture/replication#about-snitches

Cassandra Replication

http://www.datastax.com/docs/1.0/cluster_architecture/replication

Cassandra Data Partitioner

http://www.datastax.com/docs/1.0/cluster_architecture/partitioning

Cassandra Data Model

For explanation:
http://www.datastax.com/docs/1.0/ddl/about-data-model
http://www.datastax.com/docs/1.0/ddl/keyspaces
http://www.datastax.com/docs/1.0/ddl/column_family
http://wiki.apache.org/cassandra/DataModel
http://jonathanhui.com/cassandra-data-model-example

Cassandra Architecture

Links:
Cassandra Definitive Guide

Cassandra 1.0 Documentation (by DataStax)
http://wiki.apache.org/cassandra/ArchitectureOverview