How do these two different types of cells function differently in the same organism even though they both contain the same genetic instructions?

During the process of differentiation the cells effectively "reprogrammed" themselves by selectively inhibiting some genes while enabling other genes. Only the enabled genes are able to be expressed. The most common method of disabling a gene is by methylating its promoter section and/or parts of the gene itself, preventing the enzymes responsible for transcribing the gene to mRNA from binding.