What exactly is the difference between Samurai and Janissaries?

They had completely different purposes in terms of military functions. Janissaries for Turkey were one of the main components of the ground army. They took part in the battles, but in addition to everything else, they were guarding the Sultan, had police functions. Samurai are more associated with the feudal system of the Japanese state. They are similar to those Janissaries. that they guarded, but only their feudal master. They were akin to European medieval knights, not to the infantry of a later military understanding.