MongoDB is an unstructured (at LEAST, in definition) distributed document storage. For clearing purpose you can think about it as a high SCALABLE and not so high performance JSON storage (though actually it uses BSON). It has a simple but EFFECTIVE multitype index system, replication and sharding, and a lot of more nice FEATURES.

Redis is a simplest key/value STORE, with transient data caching. It can works as an efficient queue system.

Looking for more specific information you will find a lot of scenarios where both them are used, Mongo as a true persistent canonical storage, and Redis as an transient/cache storage.

MongoDB is an unstructured (at least, in definition) distributed document storage. For clearing purpose you can think about it as a high scalable and not so high performance JSON storage (though actually it uses BSON). It has a simple but effective multitype index system, replication and sharding, and a lot of more nice features.

Redis is a simplest key/value store, with transient data caching. It can works as an efficient queue system.

Looking for more specific information you will find a lot of scenarios where both them are used, Mongo as a true persistent canonical storage, and Redis as an transient/cache storage.