It's not you, it's the author. That's a horrible way to describe it.

It's partitioned: Cassandra is a clustered database that will automatically route data to the right nodes. It does this by partitioning a token ring among members of the cluster. If you need more capacity, you add nodes and they claim more of the "token ring".

The row store: Cassandra groups data within partitions (see above) which determines which hosts get the data. Within each partition, Cassandra sorts the CQL rows based on your schema. If you had a table of "employees", you could have them partitioned by last initial, and then clustered by last name - all of the employees with last name starting with "J" would be on the same machines, and then they'd be sorted on disk "Ja...", "Je...", etc