An ethnic group is a group of people who are considered to be the same in some or multiple ways. They may all have the same ancestors, speak the same language, or have the same culture, which could sometimes include religion. They often live in the same or surrounding area.
Sometimes almost all of the people in one country are of the same ethnic group, but not always. Often one country may have several different ethnic groups, or the people of one ethnic group may live in several different countries.
The International Covenant on Civil and Political Rights ensures the rights of ethnic groups in Article 27 and also gives them the right to use their own language.
A example of a ethnic group is the Romani people.