Title says it all.
Biology is kind of an all-encompassing term for all aspects of an organism (or things that may affect the organism), including its morphology, makeup, structure, anatomy/physiology, function, evolution, development, life cycle and reproduction, molecular and biochemical processes, behavior, its relationship with other organisms and its niche in its ecosystem, genetics, etc. Molecular biology is a subset of this, and deals largely with biological and biochemical processes at the molecular or cellular level–this may involve the study of cell structure, or the effect of various genes, or cell biochemistry, to name a few things. Physiology is also a subset of biology and refers to the biological functions of the various parts of an organism.
plant biology is the general scientific study of plant life
plant physiology is the study of the functioning inside of a plant
plant molecular biology refers to biology at the molecular level of an plant (i.e. DNA, proteins, sugars, etc.)
Just to add to the above comments…
Plant biology may refer just to botany, but at many universities that have an agricultural curriculum, the term also encompasses the several applied plant sciences, e.g., agronomy/crop sciences, horticulture, plant pathology, etc.