what is the difference between human genetics and molecular biology?

Human genetics describes the study of inheritance as it occurs in human beings. Human genetics encompasses a variety of overlapping fields including: classical genetics, cytogenetics, molecular genetics, biochemical genetics, genomics, population genetics, developmental genetics, clinical genetics, and genetic counseling.

Field of science concerned with the chemical structures and processes of biological phenomena at the molecular level. Having developed out of the related fields of biochemistry, genetics, and biophysics, the discipline is particularly concerned with the study of proteins, nucleic acids, and enzymes. Molecular biology has its roots in biophysics, genetics, and biochemistry. A prime focus of the field has been the molecular basis of genetics, and with the demonstration in the mid-1940s that deoxyribonucleic acid (DNA) is the genetic material, emphasis has been on structure, organization, and regulation of genes.

So according to my knowledge Human Genetics is a broad field that encompasses many other subjects. So studying Molecular Biology will be just learning another aspect of Human Genetics.

Human genetics is particularly oriented towards humans and the diseases caused by genetic defects.  This is part of the training of a first year medical student.  

Molecular biology is a broader topic because it deals with many non-human species.  Human genetics is really just a subset of molecular biology.  However, a general class in molecular biology does not spend much time with the genetic origins of human pathologies.