Zakat is an obligation ordained by Allah Most High upon every Muslim who becomes the owner of a specific amount of wealth known as the “Nisab”. If one’s wealth is equal or more than this specific amount, Zakat will become obligatory upon him.

This Nisab is the equivalent in cash, gold, silver, merchandise, etc of 612 grams of silver or 87.48 grams of gold.

Normal Zakat is obligatory on certain types of wealth, such as cash, gold, silver, business goods, livestock and agricultural produce.

On a SLIGHT tangent: http://www.sunnipath.com/resources/Questions/QA00002152.aspx

And Allah swt knows best!