What is Redis and why is it used in modern web applications?

I'm learning backend development and frequently come across Redis in tutorials and system design discussions. I understand that Redis is an in-memory data store, but I'm not sure how it differs from traditional databases like MySQL, PostgreSQL, or MongoDB.

Can someone explain what Redis is, how it works, and why it's commonly used for caching, session management, message queues, rate limiting, and real-time applications? I'd also like to know when Redis should be used alongside a database and when it can be used as the primary data store.