Learn more about Kafka records.
In Kafka, a publish-subscribe message is called a record. A record consists of a key/value pair and metadata including a timestamp. The key is not required, but can be used to identify messages from the same data source. Kafka stores keys and values as arrays of bytes. It does not otherwise care about the format.
The metadata of each record can include headers. Headers may store application-specific metadata as key-value pairs. In the context of the header, keys are strings and values are byte arrays.
For specific details of the record format, see Apache Kafka documentation.