Each human cell has 23 pairs of chromosomes. The nucleus of the cell holds these chromosomes which are made of enormous amounts of DNA. Segments of DNA make up genes which code for different proteins to be made which all have different and important functions. In some cells, certain genes are turned off and certain genes are turned on. This difference in gene functionality is what gives rise to differences between the cells.
Cells can be differentiated through signaling requiring for different types of gene expression. Almost all variety within cells with little exception, comes from the turning on and off of genes. This gives rise to different physical properties of cells even though they contain the same genome and have the same potential. Epigenetics is the study of different forms of gene expression.
Evolutionary epigenetics cause some cells through division to express as heart cells while some cells to express as skin cells and so on.
This differentiation and growth is backed up by long periods of evolution that has established inherited cues, controlled within the genes that tells it what to do. The cell wants to ensure its survival and is driven by this to proliferate. Similarly, signals from the external environment of the cell also have an impact on what the cell must do and how it must react to those situations.
Source:
Meyers, Robert Allen. Epigenetic Regulation and Epigenomics: Advances in Molecular Biology and Medicine. Weinheim: Wiley-Blackwell, 2012.