Mongo is often compared to CouchDB. But, aside from the fact that they both store JSON documents, their approaches are quite different.
Mongo stores documents in collections, which are vaguely like tables in a SQL database. Mongo queries are partial JSON documents matched against existing documents in the database. Couch builds views with map-reduce and is, in general, a little more conceptually heavy while Mongo is more straight forward with direct analogs to most SQL features.
You have not yet reached enlightenment...
From within Ruby, we can communicate with MongoDB with the mongo, bson and bson_ext gems. One fun way to learn about using MongoDB from Ruby is through the nicely kooky MongoDB_Koans, a series of unit tests with small omissions or bugs. You fix the bugs and make the tests pass, while the test harness gently urges "Please meditate on the following code...".