Hunger is the physiological drive to eat, influenced by hormones, nutrient levels in the body, and eating patterns. Scientists differentiate this from appetite, which is the psychological drive to eat. I wont discuss appetite here. Scientists also differentiate between hunger and satiety (fullness), because they are regulated by different centres. I will discuss both of these, because they directly affect each other.
The hypothalamus (HT) contains centers that tell you if you're hungry or satiated. Although they are separate centres, the hunger centers inhibit the satiety centers (i.e. when you're hungry you feel less full), and vice versa. When the hunger centers are activated, activity in your sympathetic nervous system decreases, which signals you to eat. The opposite occurs when the satiety centers are activated.
To "decide" which of these centers should be activated, the hypothalamus receives input from a few sources:
There is also evidence that orexin, a hormone involved in sleep/wakefulness cycles, stimulates hunger. Sleep deprivation has been shown to increase food intake. Relevance to weight control: Get enough sleep.