Ramadhan is the 9th month in the Muslim calendar, and to roughly 1.2 billion people, it's a very special and holy month. During this time, they devote themselves to abstinence, prayer and helping the community.
Why? Muslims fast for 3 reasons: (1) To commemorate Moses' and Jesus' 40 days of fasting as an act of cleansing in preparation for receiving the word of God (Source: http://sunatullah.com/puasa/asal-mula-puasasaum.html). (2) To cultivate compassion for others by undergoing the hardship of the poor. During the month, Muslims are encouraged to give and share generously with the community, including donating food for the poor. (3) To encourage patience and abstinence.
How? For Muslims, fasting isn't just about abstaining from food and drink. While fasting, Muslims must be patient and not lose their temper, abstain from intercourse, must not allow any object to enter any orifice of the body (activities for health reasons are exempted, e.g. brushing teeth, visiting the dentist, etc) and are encouraged to perform extra prayers. In doing so, Muslims devote themselves to each other and to God for a month and cleanse themselves of impurities.
Except Muslims are exempted from fasting for various reasons, the first and foremost is health. If, for any reason, fasting will affect your health, then you are forbidden from fasting. For example, pregnant women are not encouraged to fast as it may affect the health of the child. Muslims are also exempted from fasting during travel - as during this time they cannot follow the movements of the sun - and if their occupation is physically laborious. In these events, they will have to replace the days missed at any time before the next Ramadhan.
Effect Physically, psychologically and physiologically, people tend to feel better after fasting. Here's why: http://www.ediscoverislam.com/About-Islam/The-religion-of-Islam/why-do-muslims-fast. Personally, I find that after fasting, I feel much more cleansed and it takes much less to satisfy hunger. I feel lighter, healthier and better able to control my own body and myself. It is both a sense of achievement and a better understanding of myself that comes from more prayer, partial abstinence and patience during trying times.
Others Who Fast Fasting is not restricted to Muslims. Jews fast for 25 hours, and while Christians don't specifically have to fast, they must still abstain from something for 40 days during Lent. The Baha'is have a similar ritual of fasting for 28 days a year. Fasting is also an integral part of the Hindu religion, especially during religious festivals. In all mentioned religions, the aim of the exercise is similar: to commemorate the acts of their religious predecessors, and to purify themselves.
