In Cassandra, keys are used to uniquely identify a row in a table. Each table in Cassandra must have a primary key, which can be composed of one or more columns. The primary key can be either simple or composite.
A simple primary key consists of only one column. It uniquely identifies each row in the table.
A composite primary key consists of more than one column. The first column is the partition key, and the remaining columns are clustering keys. The partition key is used to distribute data across the nodes in a Cassandra cluster, while the clustering keys are used to order the data within each partition.