Epigenetics

Every cell in the body has essentially the same genetic sequence (the combination of A, C, G, and T nucleotides that make up a strand of DNA). On top of that sequence, there is also a biochemical program (you can think of it as your cell's operating system) that dictates how the genome is used in each cell—this is epigenetics. It decides which proteins the cell makes, how quickly it divides, whether it can transform into a different cell type (as in the case of stem cells), etc. Epigenetics is also what enables two cells with the exact same DNA to have vastly different phenotypes—what makes a neuron a neuron and a white blood cell a white blood cell.