Mongo Entity is for developers. It provides no enhancements out of the box. Modules must create custom entity types with hook_entity_info that use the included entity and controller classes to store their data in a Mongo collection.
Mongo Entity stores the entire entity as a document in a Mongo collection, including the ID, name, any properties, and all field data. It also provides subclasses for embedded entities- entities that are always stored with their parent. An example of an embedded entity might be a line item in an order- for practical purposes, the line item would never be used outside of the order. Rather than storing the line item data in its own table (or storing the fields for that line item entity type in its own collection, as the MongoDB Field Storage module does), the embedded entity is stored with its parent, in the "order" collection. Loading the order would load all of the line items, and loading a single line item would also load the order, with no extra collection queries, and minimal overhead.
The intention with Mongo Entity is to read and write complex data structures in a way that matches the model for that entity, but provides all the convenience of the Entity API.
git clone --branch 7.x-1.x email@example.com:sandbox/bartram/1974844.git mongo_entity