What are the differences between Greeks and Christians?

Some (most?) Greeks are Christians.

Greeks are the people of Greece. It is a nationality, like being American or Russian. You can be any religion and Greek.

Christians are people who believe in the religion of Christianity. Like most other religions, there are Christians all over the world. You can be any nationality and Christian.

There is a Greek Orthodox Church that I think many Greeks belong to and I believe it follows a version of Christianity that split from what is now Roman Catholic tradition.

In ancient times, the main religion was what we now think of as Greek mythology. There were also Greeks who became Jews or Christians and there were people from the Middle East lands who had converted to Christianity and went to Greece or Greek-ruled lands and therefore became Greeks.