A handy reference website
Molecular biology is really the study of larger scale molecules in living systems. It typically deals with structures that are generally larger than biochemistry aspects.
A myoglobin molecule. image: wikipedia
Topics of molecular biology generally deal with things such as proteins and enzyme structures, cell membrane structures such as interactions with DNA e.g. histones.
It provides a link to biochemistry in providing details about how some of the biochemistry relates to the function of small sections of the cell.
Here is a visualisation of the scale of molecular biology and biochemistry from a gold nano particle, antibody, proteins to salicylic acid . image: Andrew White