Zakah, which literally means purity or purification, is a wealth tax of a small percentage (usually 2.5%) for the benefit of the needy in the society. It is regarded as a debt to God and must be distributed for the pleasure of Allah to the less fortunate amongst one's relatives, and the orphans, and the needy, and the wayfarers, and the beggars, and for the freeing of slaves. (II: 2-3, II-43, II:177, IX:11, XXII:41, IX:71, III:91, XIV:91, XXIV:22, XXVI:9, XIV:35-37, XXII:26)
There are innumerable traditions of the Prophet insisting upon zakah being paid by every muslim. Imaam Ja'far Sadiq said that the one who does not give zakah can not expect his salaah to be accepted by Allah.