Imam al Ghazali writes about the following categories of fasting:
1- First level: Ordinary Fasting
this level requires abstention from three things: food, drink and sexual satisfaction. This is the minimum requirement and the reward is given accordingly.
2- Second level: special fasting
Keeping one’s ears, eyes, tongue, ahnds and feet free from sin. For such people just abstaining from the minimum requirement is not sufficient, thus they take care not to say, hear or do a wrong thing. As they restrain their organs from all kinds of prohibitions, they are rewarded accordingly.
3- Third level: Extra-special fasting
This fast involves abstaining from all unworthy thoughts, loves, desires and wishes in total disregard of everytihng except the thought and remembrance of Allah. This is the perfection of tawhid (oneness of Allah). One may physically be present in the world but the internal self is exclusively engaged towards Allah .