Its basically a datastructure store. You have a lot of data types like lists, sets, sorted sets, hash and these can directly interface with our programming languages database.
It is and in-memory database so its extremely fast and very good for high read throughput and heavy read workloads.
It is open source and free which makes it really trustworthy. It is used by us for our job server and also as a message queue and pub sub functionality for realtime notifications. Its backup and save mechanisms are really good and is easier to use than most other databases.
Its command protocol is very easy to use and implement and very fast also. Pipeling also improves threshold and is currently the fastest database I've seen.
It doesn't have truly serializable ACID transactions . Its clustering support is still not fault tolerant. It can't be used as your primary database. Doesn't have proper query language so analytics becomes a little hard and we may need to use lua for it.
If you have a lot of reads hitting your main database then you need to get redis and that stuff man.
We bring gamification to the global mass. One importart feature for gamification is leaderboards and we use redis sorted sets a lot for it. We also use it for caching our sessions so that our users get an immediate response and we don't need to hit our databases for everything.