Metaphysics is the general branch of philosophy which explores the ultimate structure / components of nature. It is helpful to remember that the name 'meta-physics' was given to Aristotle's works which, on the library shelf, came after his works on physics. Since these latter works didn't have a title the early librarians simply called them 'meta-physics,' or vaguely, 'after-physics.' Aristotle called metaphysics 'first philosophy,' and can be understood as the study of the 'ultimate principles' of reality.
Alexd181 says metaphysics is the 'ultimate study of being,' but that is really only one small part of metaphysics. That is called 'ontology,' which seeks the question as to what is fundamentally there. Heidegger, for instance, insisted that since Plato we'd totally forgotten the question of being, and it was his job to revitalize it.
Metaphysics, as a branch of philosphy, contains not only Ontology, but also Cosmology, Cosmogony (the origins of the real), Identity and Multiplicity, Free will and Determinsm, Consciousness, and the list goes on.
Numerology and astrology can be a part of metaphysics - certainly - but it is not philosophical metaphysics, from my point of view, because it does not use systematic reasoning and clear argument - two things that are central to philosophy. I don't mean to debase those things, or religion necessarily, but just want to say that they are very different ways of exploring the worlds phenomena.
Let me know what you think ;-)