what is the meaning of Datum?

Hi all,
ok this is a silly question, but I've got a doubt: what is the exact meaning
of Datum? I see having a look at the macroes (e.g., PG_RETURN_XXX) that a
Datum can be used as a pointer or as a single data, that is it can be a
reference or a value. Is this right? So for instance the fact that each stored
procedure returns a Datum means that the semantic of the Datum is interpreted
depending on how the procedure is defined in the SQL language (i.e., it returns
a integer, a tuple, ...). Am I right?

Moreover, is there a documentation (aside the source code) that explains and
links each internal data structure like HeapTuple, HeapTupleHeader, and so on?

Thanks,
Luca