The basic Ottoman Empire military structure was divided into three different structures. This consisted of the Army, the Navy and also the Air Force. You can also divide up the history of this great Empire into two main periods. The first is the classical period which covers the time from 1299 till the reforms in the early 19th century. The second period is the modern one which started in 1829 with the establishment of the Ottoman Army.
The Classical period of the Ottoman Empire military had been organized by a Turk from Western Anatolia called Osman. The infantry during that time was called yayas and the cavalry would go by musellems. Between 1444 to 1448 the Ottoman s had started to use guns like the regular rifles. The Kapikulu was the usual army which used to be commanded and also paid by the fief holders and this sect had become in a way a noble class.
Then came the Janissaries which were the infantry which consisted of the bodyguards and the household troops of the sultans. This unit would consist of the slaves as well as the war captives. Then you also had the elite cavalry in the Ottoman Empire military. There were six divisions of the cavalry and the most important one was the Sipahis. If you look at the military in medieval Europe then you will find that these look like the knights. They were founded in the reign of Sultan Murad I.
You also had the Azabs. This was the new form of the infantrymen who were irregular. Their work was to build up the roads and bridges so that the armies can be used, sometimes they would even be used as the cannon fodder so that the enemies could be slowed down. The Azabs also had branches like the Basibozuks which consisted of the criminals and the homeless who would specialise in close combat. The Ottoman Empire military would also have a military band which is actually one of the oldest varieties of the marching band.
Finally you have the modern period of Ottoman Empire military. This started in the late 18th centuries and the early 19th century. This was actually the very first attempt to transform the military forces of the Ottoman Empire into a completely modern army. In 1826 the Janissaries was disbanded in 1826 and this was known as "Vaka i Hayriye" or the good incident.