Hunger is nūr (light), satiety is nār (fire). As for shahwa (desire/appetite), it is like wood, which causes the fire to ignite. It is such a fire that you cannot extinguish it until it burns its possessor. (Qushayri, 73)
Pleasure is nothing but the perception of something that is desired and craved. Shahwa, being common to all living beings, is the arousal of the nafs (the lower self) to strive for what it desires, whether intellectually and physically in some creatures, or only physically in others, in order to attain what is desired. (Mīzān, 92)
There are two kinds of shahwa: Accidental shahwa (shahwa-i ʿāriḍī) is the false desire whose following is forbidden. Even if it is beneficial to a person one day, it is not appropriate for an intelligent person to make a habit of following it and never return. For if he does not return, the accidents present in that desire may come to dominate the intelligent person.
The other is essential shahwa (shahwa-i dhātiyya). One must follow this. Due to the softness of his nature, it is possible for a person's temperament to be reformed in this desire. In the reformation of his temperament lies his own salvation, and in the reformation of his religion lies his happiness. However, he follows it only according to the divine mīzān (balance/measure) set by the Lawgiver. This is the ruling taught by the sharia (the sacred law). In the sharia there are both rukhsah (dispensation) and ʿazīmah (strictness).