To render the fat out of duck, the fattiest cuts of meat are seared until the fat reaches its melting temperature and starts to leave the rest of the meat. As the fat melts it is spooned off and collected. To do this at home is a production, and with a fat as flavorful as duck, you could never have enough time to make as much as you could want. Duck fat is most commonly used when preparing the classic dish, duck confit, which is duck meat, submerged in liquid rendered duck fat and slowly cooked for hours and hours.
This rendered duck fat is also a more flavorful option for sautéing or frying. A teaspoon of this in your pan instead of butter when you are quick sautéing up some vegetables will be an incredible improvement from the norm. Duck fat is also healthier then a lot of other cooking fats, as it is very low in saturated fats.