Janissary about 1700
The corps of Janissaries was the main infantry unit of the Ottoman army, which had traditionally consisted mainly of cavalry. Organized in the 14th century, they were taken as boys between 7 to 14 years old from among the empire’s Christian subjects, and given an intensive education and training (which of course included conversion to Islam). They served the sultan directly—technically, in fact, they were his slaves—but were granted booty and paid a regular salary, even during peacetime (possibly the first modern standing army, in this respect). Marriage was also permitted, but when a Janissary died the corps inherited his property. Despite their “enslaved” status, Janissaries made their way into the empire’s ruling class, and even rivaled the Turkish nobility in power. Some rose so high from the ranks as to become provincial governors and grand viziers.
Red börk, 16th century
Early in the Janissaries’ history, they came under the influence of the Bektashi Sufi order, some of whom became, in effect, their chaplains. Bektash, the Sufi saint and patron of that order, lived a century before the Janissaries were established, but they were well trained in his teachings, and the most distinctive feature of their uniform, the hat called the börk, is said to imitate the sleeve of the saint, who was known for living and dressing simply. The front of the hat has a tall metal ornament that was at first a spoon, and later a fancy case that contained one. The members of a Janissary regiment ate together from a communal pot, considering themselves “brothers of the spoon,” and this became an honored token of their solidarity. As some pictures on this page show, the Janissaries also had a variety of other styles in headgear at their disposal.
Uniforms and hats distinguished Janissaries of different ranks and offices (French drawing)
Christian families were often willing to give up their sons to a life of comparative prosperity and social advancement, and eventually Muslims too began coveting admission to the Janissary ranks. In fact, by the 17th century, when the Turks conquered Crete, Christians were no longer being taken into the Janissary corps; all their recruits were Muslims.
Janissary of the 16th or 17th century
As time went on the Janissaries (at least those whose role continued to be primarily military) were not perfect servants of the Sultan. They were frequently dissatisfied with the level of compensation and, like the Roman Praetorian Guard, their demands were not easy to ignore. Most Sultans granted them raises on demand. But as the arts and technologies of warfare advanced and the Janissaries clung to their traditional ways, they ceased to be a military asset to the empire, while politically they still posed a threat to the sultan’s power. Finally, in 1826, Sultan Mahmud II announced a plan to modernize his entire army, provoking (as the sultan probably intended) a mutiny that ended in the bloody defeat and disbandment of the Janissary corps. (This event is known in Turkish history as the Auspicious Incident.)
17th-century Janissaries
Janissaries had come to Crete with the first wave of the Turkish invasion, and they built their Chaniá mosque in 1645, the same year the city fell to the invaders. The Rough Guide describes them as mercenaries under the control of the Turkish landlords, and one book suggests that they may have been sent to Crete mainly to get them away from Constantinople. If this is more than speculation, it may be that the landlords took over the Sultan’s obligation to pay the salaries of those Janissaries who helped them win and keep the island.

Starting at about the same time as the conquest of Crete, however, requirements for membership in the Janissary corps were being weakened throughout the empire because of pressure from the many citizens (of all faiths, including Islam) who coveted the educational, economic, and political advantages they could gain as Janissaries. Many Cretan Greeks changed religions to make this move. The Janissaries seem to have become integrated into the local economy, rather than remaining separate as they had in the past. I don't know if the policy that designated the corps as the sole inheritor of every Janissary's estate remained in effect, but I wouldn't be surprised if that was weakened also.
American Janissary re-enactors
Odd as it may sound, the US is home to at least one group of Janissary re-enactors. According to their website, the members are mostly of Slavic descent, although some have Turkish ancestry. The group's scope appears to take in quite a bit more than the Ottoman empire; they are interested in all the sides in all the wars fought by the Polish-Lithuanian Commonwealth in the 16th and 17th centuries.

Busy as they must be, I owe them thanks for the sketches you'll see if you look at the Janissary fashion show below. They don't say where they got the pictures; the style looks Turkish, but there is Western handwriting (probably German) on the pages.