Who were the Mamluks?
Since you asked about the Abbasids before, I assume you are asking about the Mamluks from that same time period. The history of the Mamluks begins with the Abbasids. The Mamluks were slaves that the Abbasids bought. They came from Turkey and the Caucasus.
What made the Mamluks special is that they fulfilled a very different role than regular slaves did. The Mamluks were used especially as soldiers. This is very different because slaves are seldom trusted with weapons. The Mamluks were very useful to the Abbasid caliphs because they had no loyalties to anyone else within the Abbasid society. They were not like typical warriors who had more allegiance to their own tribes than to the Abbasids. Therefore, the Abbasids valued them highly.