끓이다 refers to the liquid you are boiling. "물을 끓이다" "To boil water." 삶다 refers to cooking something by putting it into something boiling. "고기를 삶다" "To cook meat (by putting it in boiling liquid)". 찌다 refers to cooking by boiling liquid underneath and using the steam to cook. "만두를 찌다" "To cook dumplings (by placing it above boiling water)"
굽다 usually refers to cooking something by directly putting it on a source of heat. Usually it refers to cooking meat or fish on a frying pan. 튀기다 implies that there is extra oil being used to fry the food. This is the word closest to "frying" in English. 볶다 refers to cooking a combination of ingredients together while mixing them. Think of what you do when you make fried rice.