Topics: Kafka scales topics by spreading them over many partitions. Each partition can be placed on a separate broker.
Producers: By having multiple partitions of a topic, Kafka producers can publish a huge number of messages to that topic. Multiple producers can publish messages on the same topic.
Consumers: Kafka scales consumers by partition such that each consumer gets its share of partitions. A consumer can have more than one partition, but a partition can only be used by one consumer in a consumer group at a time.
Broker: Adding or removing brokers is very easy. Whenever a new broker is added or removed, Kafka rebalances topic partitions. Kafka tries to distribute leaders and followers of a partition on separate brokers.