Redis is an in-memory (with persistence) key-value database that also implements some basic structures like lists, sets and hashes natively.

Kafka is a distributed logging system that can ingest large amounts of data straight to disk, then allows for multiple consumers to read this data through a simple abstraction of topics and partitions. Consumers maintain their own position of where they last read up to (or re-read things if they want) and everything is sequential I/O which creates very high throughput.