This project repo goes through a very quick tour of Morphia
Morphia is a Java Object Document Mapper for MongoDB.
MongoDB is a document-oriented NoSQL database which it stores its data in a collection of 'Documents'. Java represents its domain data in Java Objects. Objects and Documents are 2 different things and their structure are obvious different. Hence, there is a need to map or 'convert' between these 2 types of entities.
Therefore, an ODM exists. ODM is Object Document Mapping. Morphia is one implementation of an ODM for Java. There are others out there. It looks like the official ODM as its from MongoDB itself. Others are: MongoJack
ODM is analogous to ORM in Object - Relational Mapping.
ORM exists in traditional Relational Databases to Java Objects conversion.