1. The API allows applications to send streams of data to topics in the Kafka cluster.
2. The Streams API allows transforming streams of data from input topics to output topics.
3. The API allows managing and inspecting topics, brokers, and other Kafka objects.

Kafka exposes all its functionality over a language independent protocol which has clients available in many programming languages. However only the Java clients are maintained as part of the main Kafka project, the others are available as independent open source projects. A list of non-Java clients is available here.