Zakat is the one of the fundamental pillar of Islam with out which a person can not called him or her self a Muslim. As every Muslim believes that every thing belongs to GOD and the wealth is just been to human as a trust to give it back in the way of GOD. The word Zakat means both 'purification' and 'growth' Zakat is the amount of money that every adult, mentally stable, free, and financially able Muslim, male and female, has to pay to support specific categories of people.
This category of people is defined in surah at-Taubah (9) verse 60: "The alms are only for the poor and the needy, and those who collect them and those whose hearts are to be reconciled, and to free the captives and the debtors, and for the cause of Allah, and (for) the wayfarers; a duty imposed by Allah. Allah is knower, Wise." (The Holy Qur'an 9:60).
It is agreed between Muslims in all the centuries the obligatory nature of paying Zakat for gold and silver, and from those the other kinds of currency, Islam has set some standard for paying ZAKAT which is defined by many Islamic scholars and Islamic ulemah with the guidance of hadith and Quran according to hadith and Quran Zakat is obligatory when a certain amount of money, called the nisab is reached or exceeded. Zakat is not obligatory if the amount owned is less than this nisab. The nisab (or minimum amount) of gold and golden currency is 20 mithqal, this is approximately 85 grams of pure gold. One mithqal is approximately 4.25 grams. The nisab of silver and silver currency is 200 dirhams, which is approximately 595 grams of pure silver. The nisab of other kinds of money and currency is to be scaled to that of gold, 85 grams of pure gold. This means that the nisab of money is the price of 85 grams of 999-type (pure) gold, on the day in which Zakat is paid. (Current Gold Prices)
Zakat is due on every adult Muslim after a time span of one lunar year passes with the money in the control of its owner. Then the owner needs to pay 2.5% (or 1/40) of the money as Zakat. (A lunar year is approximately 355 days).