Kafka is a message passing system, messages are events and can have keys.
Kafka replicates partitions across the cluster for fault tolerance and message durability. 
Topics are streams of messages of a particular category.
Producers are client processes that send messages to a broker on a topic and partition.
Partitions are append only, ordered logs of a topic’s messages. Messages have offsets denoting position in the partition.
A Kafka cluster is made up of brokers that run Kafka processes. 
Consumers read messages from topics' partitions on brokers, tracking the last offset read to coordinate and recover from failures. Consumers can be deployed in groups for scalability.