Wikipedia
Devşirme (literally "collecting" in Turkish), also known as the blood tax or tribute in blood, was chiefly the annual practice whereby the Ottoman Empire sent military officers to take boys, ages 8 to 18, from their families in order that they be raised to serve the state. This tax of sons was imposed only on the Christian subjects of the empire, in the villages of the Balkans and Anatolia.
The boys were then forcibly converted to Islam with the primary objective of selecting and training the ablest children and teenagers for the military or civil service of the empire, notably into the Janissaries. Started in the mid 1300s by Murad I as a means to counteract the growing power of the Turkish nobility, the practice itself violated Islamic law.
By 1648, the practice was slowly drawing to an end. An attempt to re-institute it in 1703 was resisted by its Ottoman members who coveted its military and civilian posts. Finally in the early part of Ahmet III's reign, the practice of devşirme was abolished.