The two document types are Document and EmbeddedDocument. They are made up of Plugins. Also built-in is a simple type system that supports all of Mongo’s types out of the box and allows you to add your own.
MongoMapper 0.7: Identity Map - A man with two watches never knows the time. Learn how to use the identity map so your objects only get loaded from the database once.
A Nunemaker Joint - A GridFS plugin for MongoMapper. Good example of plugins and of how to work with GridFS.