Although their somewhat flamboyant uniforms resemble those of the Yeniçeri Corps , these Nizamları Corps or "New Model Corps" are in fact the Ottoman answer to European Musketeers and continue to remain the mainstay of the Ottoman Army until Mechanisation is achieved and you can attempt to obtain Askeri to replace them. Nizamları are a most welcome addition to the Ottoman arsenal which until the late game has to put up with their costlier and more inefficient predecessors which cannot even be used to assault buildings. Even so, although they are clearly tougher than even Western Musketeers they are still quite costly and don't train as quickly, so as an Ottoman player you should be wary of this issue and try to hold them back should you see cavalry. As these units are also rather costly, try to defray the cost of using them by supplementing them with Swordsmen to fend off enemy line infantry should you see them.
The Janissaries (Ottoman Turkish: يڭيچرى yeniçeri, meaning "new soldier") were elite infantry units that formed the Ottoman Sultan's household troops and bodyguards, but by 1620 they were hereditary and corrupt and an impediment to reform. In their place, reforms were sought to shore up declining Ottoman strength, collectively known as the Nizam-e-Cedit or "New Order". The French Republic, seeking to renew its relations with Turkey with an eye towards controlling the eastern Mediterranean and forestalling British intentions in the Far East, sent delegations of military advisors to the sultan Selim III.
Under their supervision, a new system of fire used by the French based on the three-rank 'peloton (company)' firing system was introduced, and was used to create a 100-strong "Bostanci" or "Gardener" regiment (named so because it was either under the Sultan's supervision or simply because of the proletarian origins of its members). The new units raised under the French reforms weren't exactly up to scratch and many wars later with the French, Russians, and Greeks would show just how weak they were, but they still managed to be of significant assistance to British efforts in retaking Egypt during the Napoleonic Wars.