this is good stuff to read;
The origins of the Janissaries are shrouded in myth though traditional accounts credit Orhan I – an early Ottoman bey, who reigned from 1326 to 1359 – as the founder.[3] Modern historians, such as Patrick Kinross, put the date slightly later, around 1365, under Orhan's son, Murad I, the first sultan of the Ottoman Empire.[1] The Janissaries became the first Ottoman standing army, replacing forces that mostly contained tribal warriors (ghazis) whose loyalty and morale were not always guaranteed.[1] From Murad I to 1648, the Janissaries were gathered through the devşirme system. This was the recruiting of non-Turkish children, notably Balkan Christians; Jews were never subject to devşirme, nor were children from Turkic families. In early days, all Christians were enrolled indiscriminately; later, those from Albania, Greece, Bosnia, Serbia and Bulgaria were preferred.[4][5]
The Janissaries were kapıkulları (sing. kapıkulu), "door servants" or "slaves of the Porte", neither free men nor ordinary slaves (Turkish: köle).[6] They were subject to strict discipline, but they were paid salaries and pensions on retirement, and were free to marry; those conscripted through devşirme formed a distinctive social class[7] which quickly became the ruling class of the Ottoman Empire, rivaling the Turkish aristocracy in one of the four royal institutions: the Palace, the Scribes, the Religious and the Military. The brightest of the Janissaries were sent to the Palace institution (Enderun), where the possibility of a glittering career beckoned.
According to military historian Michael Antonucci, every five years the Turkish administrators would scour their regions for the strongest sons of the sultan's Christian subjects. These boys, usually between the ages of 10 and 12, were then taken from their parents and given to the Turkish families in the provinces to learn Turkish language and customs, and the rules of Islam; these boys were then enrolled in Janissary training. The recruit was immediately indoctrinated into the religion of Islam. He was supervised 24 hours a day and subjected to severe discipline: he was prohibited from growing a beard, taking up a skill other than war, or marrying. The Janissaries were extremely well disciplined (a rarity in the Middle Ages).
Greek Historian Dimitri Kitsikis in his book, Türk Yunan İmparatorluğu ("Turco-Greek Empire")[4] states that many Christian families were willing to comply with devşirme because it offered the possibility of great social advancement. Conscripts could one day become Janissary colonels; statesmen who might one day return to their motherland as governor; or even Grand Vizier or Beylerbey (governor general), with a seat in the divan (imperial council).
Perhaps the most famous Janissaries were George Kastrioti Skanderbeg, son of a despot in northern Albania who later defected and led a 20-year Albanian revolt against the Ottomans, and Sokollu Mehmed Paşa, a Serbian peasant from Bosnia who later became a grand vizier, served three sultans, and was de facto ruler of the Ottoman Empire[8] for more than 14 years.The Ukrainian, Bulgarian and Serbian languages employed the name Janissary to refer to any warrior who converted from Christianity to Islam.