Cassandra is architected in a peer-to-peer fashion and uses a protocol called gossip to communicate with other nodes in a cluster. The gossip process runs every second to exchange information across the cluster.
Gossip only includes information about the cluster itself (up/down, joining, leaving, version, schema, etc.) and does not manage the data. Data is transferred node-to-node using a message passing like protocol on a distinct port from what client applications connect to. The Cassandra partitioner turns a column family key into a token, the replication strategy picks the set of nodes responsible for that token (using information from the snitch) and Cassandra sends messages to those replicas with the request (read or write).