There is a difference between your physiological need for food (hunger) and the psychological factors that prompt you to eat (appetite). Hunger is the physical need for nourishment that drives you to consume food. Once you start eating, hunger will subside as the feeling satiation begins to set in. Satiation will determine how long and how much you eat. Satiety determines the length of time between eating episodes. An increase in satiety will delay the start of your next meal or snack.
Your appetite is affected by hunger, as well as environmental factors, like seeing or smelling something you think will taste good, your social setting, your stress level, and so forth. Hunger, in turn, is affected by many physiological mechanisms, as well as genetics. Both hunger and appetite ultimately affect what you eat.