MapReduce is a flexible and powerful alternative to declarative query languages like SQL that takes advantage of Riak's distributed architecture. Dan Reverri covers basics and best-practices for MapReduce querying in Riak.
Sometimes you need more than words to illustrate a point. Here is Basho's humble attempt to clarify the difference between "Dynamo-Style" systems (like Riak) that use consistent hashing to achieve fault tolerance and prevent data loss, and systems that use techniques like sharding.
Ruby on Rails is a powerful web framework that focuses on developer productivity. Riak is a friendly key-value store that is simple, flexible and scalable. Put them together and you have lots of exciting possibilities!