Under Islamic law, the jizya was a per capita tax levied on a section of an Islamic state's non-Muslim citizens. Under the law a distinction is made between two categories of non-Muslim subjects - pagans and dhimmis. The dhimmis being “protected peoples,” or “peoples of the book”, that is, people whose religious beliefs are based on sacred texts, such as Christians and Jews. The dhimmis were tolerated by Muslim rulers and were allowed to practice their faith and enjoy a measure of communal autonomy, subject to the special head tax known as the jizya and, thus, receive protection from the Muslim state.
The rate of taxation and the means of collection of the tax varied greatly from province to province. These rates were influenced by local pre-Islamic customs. The tax receipts were to be used, in theory, for charitable purposes and payment of salaries and pensions. However In practice, the revenues received from the jizya were deposited in the private treasuries of the rulers. The Ottoman Turks usually used the proceeds of the jizya to pay military expenses.
In theory, converts to Islam were no longer required to pay the jizya. However, the Umayyad caliphs demanded the jizya from recent converts to Islam as well as from the dhimmis when faced with financial difficulties during the eighth/ninth centuries. This discrimination against converts caused the Abū Muslim rebellion in Khorasan in 747, precipitating the downfall of the Umayyads.
The Arabic term jizya appears in verse Qur'an 9:29, but the Qur'an does not specify jizya specifically as a tax per head. According to Paul Heck in the Encyclopedia of the Qur'an, the jizya seems to be a developed form of the Sassanian practice of taxation.