I'm not absolutely sure, maybe one of our members with more recent experience of academia can clear this up, but I would have said Maestr?a (Masters) is more common on this side of the pond, while Posgrado (post-graduate) is a more European expression.
How do I say master degree in Spanish?
In many countries, including the DR if I am not mistaken, you can take a pos-grado that is not a full degree. In the States it is the equivalent of a certificate course.