Every cell in your body, barring things like cancer, generally has the same DNA. Think of this like a big instruction manual telling each cell what to do and what to make. However, not all of your cells are the same - some are muscle cells, some are fat cels, some are neurons, etc. To control what cells do what, is the role of gene regulation, or epigenetics.

Think of it like this - every cell in the body has the same big manual, but different cells have bits crossed out or vigorously circled to explain what they're meant to do, so certain genes are more or less expressed. That's how different cells get differentiated. The thing is that this gene expression can be altered by things you do in life, and this particular gene expression pattern can be passed down to children - some other examples include increased epigenetic propensity towards obesity from children born to obese parents.