I don't understand whether "RDF", "RDF Model", and "RDF Data Model" are describing different concepts or no, and if yes, what are the differences.
For instance, in this W3C document (under section 2.1) it is mentioned:
The foundation of RDF is a model for representing named properties and property values. The RDF model draws on well-established principles from various data representation communities. RDF properties may be thought of as attributes of resources and in this sense correspond to traditional attribute-value pairs. RDF properties also represent relationships between resources and an RDF model can therefore resemble an entity-relationship diagram. (More precisely, RDF Schemas — which are themselves instances of RDF data models — are ER diagrams.) In object-oriented design terminology, resources correspond to objects and properties correspond to instance variables. The RDF data model is a syntax-neutral way of representing RDF expressions. The data model representation is used to evaluate equivalence in meaning. Two RDF expressions are equivalent if and only if their data model representations are the same. This definition of equivalence permits some syntactic variation in expression without altering the meaning. (See Section 6. for additional discussion of string comparison issues.)
In the above case, do they use "RDF data model" and "RDF model" interchangeably or do they refer to different things?
Or in this W3C document it is mentioned:
RDF uses the following key concepts: Graph data model […]
Or somewhere else it is mentioned:
The underlying structure of any expression in RDF is a collection of triples
Does "RDF" here mean the model? Data model? Or all 3 mean the same thing?
The first link is to the 1999 version of RDF (sometimes called "RDF 1.0").
The foundation of RDF is a model
so we are talking about the general concepts.
In RDF 1.0, the only syntax is RDF/XML (N-Triples was only to support the test cases). The language in the documents reflects this.
A "data model" is the general term for the data elements and structures used to represent the concepts.
In 2004 RDF 1.1, multiple syntax forms were introduced (Turtle, N-triples etc). There is a section "Graph data model" in RDF 1.1. With multiple syntaxes, it was more important to differentiate general concepts, the RDF data model and the concrete syntaxes. RDF 1.1 also downplays the use of "RDF model" and emphasises the idea of the data model.