Redis and Memcached are both open-source, in-memory key-value data stores used for caching, but they differ in features and use cases. Redis stores data in memory and can persist to disk, supports data types beyond strings like lists and sets, and allows replication. Memcached only stores data in memory and does not support persistence or replication. While both are fast, Redis is more full-featured but requires more memory, making Memcached better for simple caching use cases with high traffic.