As far as I understand it, it is someone who sees themselves as mostly, but not completely, as a boy/girl. They can be either biological sex.
Maybe this helps: http://emptyclosets.com/forum/gender-identity-expression/150966-androgyne-identity-2.html#22 hugs
This about covers it really. I've known demi-girls that are biologically male and demi-girls that are biologically female, so in that respect demi-genders aren't really a trans-specific thing.