Linguistically zakat means growth and increase and it is used for the increase in crops and property. In the Shari'a it is a term used to designate the amount of money or goods taken from specific property when it reaches a specific amount at a specific time which must be spent in specific categories with specific ways. It is called zakat because the one who gives it grows by doing it with Allah Almighty, i.e. his rank with Him is raised by that. It is obligatory to pay zakat on money, crops and livestock. Crops consists of food groups used for life in general.
Livestock refers to camels, cattle, and sheep and goats.
Zakat became an obligation in the second year of Hijra. The evidence for its obligatory nature is the Book, Sunna and consensus. Whoever disputes its obligatory nature is an unbeliever. Ibn Habib says he is an unbeliever, but this is unlikely. There are preconditions for the obligation and preconditions for the validity.