Biology is the study of living things ("bio" = life, "ology" = the study of). Cell biology is the study of the cells of living things. Like the atom is the basic unit of matter, the cell is the basic unit of all living things. Cell biology may study the life cycle of a cell, cell division, the formation of cells, organelles within a cell, types of cells, the structure of cells, and/or the function of a cell.
Sometimes the term "molecular biology" is used interchangeably with the term cell biology. Like cell biologists, molecular biologists may be concerned with the entirety of a cell. However, a molecular biologist most often deals with the components within a cell. Thus, most techniques used by a molecular biologist require that a cell be split open. For example, a molecular biologist may deal with the expression of genes.