ORM or Object-Relational Mapper maps objects to relational database records (also sometimes referred to as relations, hence the name). Oracle, MySQL, and PostgreSQL are well-known relational databases.
ODM or Object-Document Mapper does the same for non-relational, document-based databases like MongoDB - it will map objects to documents.
In both cases the idea is to make persistence of objects in an object-oriented application as simple and transparent as possible - whatever kind of target database system you may be using for that persistence.
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