There are three levels of fasting, the general fast, the specific fast and the more specific fast.
As for the general fast, then it is the refraining of ones stomach and their private parts from fulfilling their desires.
The specific fast is the refraining of ones gaze, tongue, hands, feet, hearing and eyes, as well as the rest of his body parts from committing sinful acts.
As for the more specific fast, then it is the heart’s abstention from its yearning after the worldly affairs and the thoughts which distance one away from Allaah, as well as its (the heart’s) abstention.
From all the things that Allaah has placed on the same level.
From the characteristics of the specific fast is that one lowers his gaze and safeguards his tongue from the repulsive speech that is forbidden, disliked, or which has no benefit, as well as controlling the rest of his body parts. . In a hadeeth by al-Bukhaaree:
‘‘Whosoever does not abandon false speech and the acting upon it, Allaah is not in need off his food and drink.’’ 
Another characteristic of the specific fast is that one does not overfill himself with food during the night. Instead, he eats in due measure, for indeed, the son of Aadam does not fill a vessel more evil than his stomach. If he were to eat his fill during the first part of the night, he would not make good use of himself for the remainder of the night. In the same way, if he eats to his fill for suhoor, he does make good use of himself until the afternoon. This is because excessive eating breads laziness and lethargy therefore, the objective of fasting disappears due to one’s excessiveness in eating, for what is indeed intended by the fast, is that one savours the taste of hunger and becomes an abandoner of desires.
From : The Inner Secrets of Fasting
Imam Muwaffaq-ud-Deen Ibn Qudamaah Al-Maqdisee