Epigenetics refers to the study of the changes in organisms caused by modifications of gene expressions, instead of changes in the genetic codes themselves. So while epigenetics doesn't change the genes, it does affect how genes are read by cells, and therefore how they make proteins. Although scientists are still figuring out how this works in humans, we know that epigenetic "tags" can essentially be turned on or off by certain circumstances, such as what you eat, where you live, who you interact with, when you sleep, how you exercise, and even as you get older.
The classic example of epigenetics shown in humans highlights how young boys who suffered through the Swedish famine later in life had sons who were much healthier than the sons of those boys-turned-men who had not lacked for food during the famine. In fact, the evidence for this influence was so strong that it suggested that the sons of boys who did not have enough to eat during the famine lived an average of 32 years longer than the sons of the better-fed boys. Why is this? Well, unfortunately we haven't quite figured that part out yet, because understanding epigenetics means dealing with more than 20,000 genes!