NoSQL systems are not SQL, but this does not mean they are unable to model relations. For instance in Redis you often have something like:

    user:1000

That is an hash with a few attributes, and among this attribs you can have 'group_id", that is an integer, for instance 50. Then if you access

    group:50

You have another data structure representing the group the user is associated with.

I believe the de-facto nomenclature for a NoSQL database's equivalent of an ORM is an "ODM", Object Document Mapper.

Good point. Probably should have just called it a mapper.