Kafka is a high throughput publish-subscribe messaging system implemented as distributed, partitioned, replicated commit log service.

Taken from official Kafka site

Fast

A single Kafka broker can handle hundreds of megabytes of reads and writes per second from thousands of clients.

Scalable

Kafka is designed to allow a single cluster to serve as the central data backbone for a large organization. It can be elastically and transparently expanded without downtime. Data streams are partitioned and spread over a cluster of machines to allow data streams larger than the capability of any single machine and to allow clusters of co-ordinated consumers

Durable

Messages are persisted on disk and replicated within the cluster to prevent data loss. Each broker can handle terabytes of messages without performance impact.

Distributed by Design

Kafka has a modern cluster-centric design that offers strong durability and fault-tolerance guarantees.