Redis is a keystore . Usually you can insert a primitive value (int, string, bool) or an array of primitives under one key. Data retrieval is usually limited to a key request. These are the most basic NoSQL databases.
Kassandra is a repository for columns . It looks like a store of key values, but supports the embedding of key-value pairs up to four levels in depth. Requests are limited to the query using the keys and map reduction functions. This type of database has a rather complex data model ( does "supercolumn" ring any calls? ) And is very specialized for extremely large amounts of data.
MongoDB and CouchDB are document databases . They both store JSON documents that are not schema-limited, giving you more flexibility. The database allows you to query the contents of these documents, which makes it very easy to retrieve data compared to other types of NoSQL databases. Card reduction features are also supported.
Martin Fabik's answer provides good links to comparisons between MongoDB and CouchDB.
Ayende has a series of blog posts called This Is Not SQL Thing that handles each of these types. This is a good introduction to the NoSQL concept, the various types of NoSQL databases, and how to query each one. I highly recommend you read his articles on the types of databases that I mentioned above, they are very instructive!