Ethnicity and religion were very much tied together. Muslim was the dominant belief system, but with toleration to both Judaism and Christianity. Orthodox Christians, Armenians (Apostolic, Catholic, and Evangelical), Syriac Orthodox...plenty of diversity, even within those three major religions. It's interesting to note that both the Jewish and Christian minority populations were allowed into every level of society from military to government. A list of the largest ethnic groups would include the Turks, Arabs, Greeks, Slovenians, Serbs, Albanians, Croatians, Armenians, Kurds. As you read more, you'll get a feel for the fact that diversity was one of the most notable things about the Ottoman Empire.
Unlike many of the empires in the Middle Ages and in the early modern period, the Ottoman Empire took a very open and welcoming view of different nationalities and religious groups. Tracing its beginnings to the fourteenth century, the Ottoman Empire allowed the indigenous inhabitants to practice their own religions, provided they pay a tax to the government. For this reason, religious groups who had faced persecution in Europe actually made the journey to live in the Ottoman Empire. In the Ottoman Empire, one would find a significant Muslim population - ultimately the official religion of the empire; in addition, a large number of Greek Orthodox and a smaller number of Catholics lived within Ottoman borders; after the Protestant Reformation in the sixteenth century, one could also find Protestants within the Ottoman Empire.
What allowed for such a plurality of religions to coexist under a single imperial power was the sheer breadth of the Ottoman Empire. At its greatest extent, it covered an area from just outside Vienna in the northwest, Egypt in the South, the Russian steppes in the northeast, and eastward into modern-day Iran. Ultimately, one would find Byzantines, Turks, Austrians, various Slavic peoples, Syrians, Georgians, Armenians, Persians, Egyptians, Arabians, and a number of smaller populations inhabiting Ottoman lands.