A single cache server has limited memory. Distributed caches spread data across multiple servers.
Redis: In-memory data store. Supports strings, lists, sets, sorted sets, hashes. Persistence options. Single-threaded but fast.
Memcached: Simpler than Redis. Just key-value strings. Multi-threaded. Good for simple caching.
Both use consistent hashing to distribute keys across nodes. If you add or remove a node, only of keys need to move.
In interviews, default to Redis. It's more versatile and what most companies use.