Embedded documents are almost identical to Documents with one exception: they are saved inside of another document instead of in their own collection. For example, lets say we have orders and orders have line items.
By including EmbeddedDocument instead of Document, MongoMapper knows to store the line items inside of the orders. In Mongo, this would create a collection named orders with one document that contained 3 line items. Below is what the document would look like represented in Ruby.