In Cassandra, rows of data are stored in tables based on the hashed value of the partition key, called a token. Each node in the cluster is assigned multiple token ranges, and rows are stored on nodes that are responsible for their tokens.
Each keyspace (collection of tables) can be assigned a RF. The RF designates how many copies of each row should be stored in each data center. If a keyspace has a RF of three, then each node is assigned primary, secondary, and tertiary token ranges. As data is written, it is written to all of the nodes that are responsible for its token.