(I work on MongoDB, but trying to give a balanced opinion.)

Redis is a great key-value store, MongoDB is more of a fully-featured database. Redis has some nice set operations and is pretty easy to learn (all of the commands are here: http://redis.io/commands). MongoDB is also pretty easy to learn (click the "Try it out" button at http://mongodb.org/), but there are a lot of advanced features to learn about.

So, if you need a key-value store, Redis is a great choice. If you want to do something more complex, MongoDB would probably work better.