Materials Science is the study of materials that engineers
are interested in working with.  The actual subject matter
has a large degree of overlap with solid 
chemistry and
applied 
physics, and also the science of 
geology, plus of
course the engineering diciplines themselves.  The existence
of materials science as a separate discipline is more a
mater of history than anything inherent in the subject itself
(much like, say, the split between 
anthropology and 
sociology):
when engineers got interested in things besides 
metal,
the metallurgists became "materials scientists".