Hi Paul,
A hybrid is the offspring (usually infertile) of parents which are of two
distinct species, and usually shows some morphological characteristics of
both parent species.
An intergrade is simply a transitional form between two morphologically
different forms of the same species.  In fact, the existence of intergrades
is often taken as evidence that two otherwise distinct forms are not in fact
separate species.  For example, if specimens from a northern locality have
very heavy, tall-spired shells, and specimens from a more southern locality
have light-weight, low-spired shells, someone might postulate that they are
separate species.  However, if there are medium-weight, medium-spired
specimens in intermediate localities, that would suggest that there is only
one species, with a gradual transition from one morphological form to the
other.
Regards,
Paul M.