Multicellular organisms

I asked one question today which, even, my teacher couldn’t answer properly. My question was : “All the cells from a multicellular organism is derived from a single cell therefore they have exactly same genes. Then, how come cells take up specialized tasks. Such as neurons behave differently than skin cells even though both have exactly same genes.”

I know that some genes activate while some deactivate in stages from zygote to adult. But how exactly it is done? Is there any external influences?

To be precise: “How do stem cells know when to specialize and what type of cell it has to become?”