Some scholars said, "There are three categories of Siyaam (fasting): Sawm (fasting) of common people, Sawm of the believers and Sawm of the pious." The Sawm of common people is abstaining from eating and drinking, and the Prophet, peace be upon him, referred to the Sawm of common people in the following Hadith:
(Many of those who fast get nothing from their fasting but hunger and thirst)
The Sawm of common people is but one of their habitual activities, and they do not know the purpose of their Siyaam. They continue with the same bad habits in their lives. They keep sinning, they continue their mixing with women whom, they are forbidden from mixing with, holding parties where men and women are mixed, listening to forbidden things and violating Allah's Shari'ah rulings. They sin while they are observing their Siyaam!
This is how common people observe Siyaam. They just refrain from eating and drinking, and they get nothing but hunger and thirst, so their Siyaam is pointless, and they get nothing worthwhile from it in return. Such Siyaam is not observed in the way Allah, Exalted and Sublime be He, wants, and it does not achieve its intended purpose. This is the Siyaam of common people.