According to Qur'an, can you please let me know the exact definition of the following people ? 1. Muslim 2. Jew 3. Christian 4. People of the book 5. Believers 6. Kaafir.
Muslim: Muslim has two meanings in the Quran. The first one is what we understand by the word “Muslim”, someone who follows Prophet Muhammad 
and believes in the Quran. The second meaning refers to anyone who follows any of God’s true prophets. For example, Prophet Ibrahim is also described as a “Muslim”.
Jew: The Quran uses two different terms for Jews. The first one is Banū Isrāʾīl (“Children of Israel”), which refers to genetic Jews, meaning Jews descended from other Jews–whether they are religious or not, whether they follow Judaism or another religion. The second term is yahūd, which refers to Jews who identify with Jewish culture and religion. So a Jew who converts to Islam is still a member of the Children of Israel, but they are no longer a member of yahūd. In English the word “Jew” mixes up both meanings, but the Quran uses this superior terminology which helps make a distinction between the Jewish race (Children of Israel) and the Jewish culture-religion (Judaism, yahūd).
Christian: A Christian is a person who believes Jesus to be the Messiah and who follows the Gospels.
People of the Book: People who follow one of God’s true pre-Islamic revelations (whether the revelation has been corrupted or not). These include Jews, Samaritans, Christians and Sabians (followers of Mandaeism).
Believers: Anyone who believes in the true God regardless of their religion.
Kafir: A person who believes in God deep in their heart, knows His revelation is true, yet rejects it out of arrogance, greed or some other base desire. The exemplary kafir is Satan, who believes in God, has stood in God’s presence, yet rebels against God. For more on this please see this previous answer.