官网的解释是

A key/value pair to be received from Kafka. This consists of a topic name and a partition number, from which the record is being received and an offset that points to the record in a Kafka partition.

kafka0.9客户端都以record为一条消息进行发送，record包含一个键值对，分区和topic名。key像map中的key，只是一条record的一个传递属性，未来它将用于去除重复发送的数据。