Redis is a **key-value store**. You can usually insert a primitive value (int, string, bool), or an array of primitives under a single key. Retrieval of data is usually limited to query by key. These are the most basic NoSQL databases.
Cassandra is a **column-family store**. It's similar to a key-value store, but supports nesting of key-value pairs up to about four levels deep. Querying is limited to query by key and map-reduce functions. This type of database has a rather difficult data model ([does 'supercolumn' ring any bells?](
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)) and is highly specialized for extremely large amounts of data.
MongoDB and CouchDB are both **document databases**. They both store JSON documents, which aren't restricted by a schema, giving you a lot of 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. Map-reduce functions are also supported.
Martin Fabik's answer contains some good links to comparisons of MongoDB and CouchDB.
Ayende has a series of blog posts called [That No SQL Thing](
http://www.google.com/search?q=site%3Aay...l+thing%22) that handles each of these types. It's a good introduction to the NoSQL concept, the different types of NoSQL databases and how to query each of them. I highly recommend you read his articles on the database types I mentioned above, they are very instructive!
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