In Apache® Kafka®, a topic is a stream of "related" messages. The stream is like a log, or sequence of messages. Application developers define the topics and Kafka producers write messages to topics.

There is a many-to-many relation between producers and topics. One, or many, producers can write to one, or many, topics.

A topic name, or stream name, refers to the destination of the message.

Message streams are persistent in Kafka. This means that messages do not disappear once received. This is in contrast with classic "pub-sub" systems such as JMS, which as soon as the message is received by the subscriber, is removed from the system. In Kafka, message retention periods are configurable, usually based on a length of time or the size of the underlying storage.