Cassandra is a partitioned row store, where rows are organized into tables with a required primary key.
The first component of a table's primary key is the partition key; within a partition, rows are clustered by the remaining columns of the PK. Other columns may be indexed independent of the PK.
This allows pervasive denormalization to "pre-build" resultsets at update time, rather than doing expensive joins across the cluster.
A blog post by committer Tyler Hobbs that gives Basic Rules of Cassandra Data Modeling.
DataStax has a good introduction to data modeling in Cassandra here.
For more detail, see Patrick McFadin's data modeling series: