A nation may refer to a community of people who share a common language, culture, ethnicity, descent, and/or history.  In this definition, a nation has no physical borders. However, it can also refer to people who share a common territory and government (for example the inhabitants of a sovereign state) irrespective of their ethnic make-up.

In international relations, nation can refer to a country or sovereign state.  The United Nations, for instance, speaks of how it was founded after the Second World War by “51 countries” and currently has 193 member states. The word nation can more specifically refer to a tribe of North American Indians, such as the Cherokee Nation.

A county irs a geographical region. A country may be the territory of a sovereign state, the territory of a non-sovereign (or formerly sovereign) political division, or a region associated with a certain people or certain characteristics. Sometimes it is used to refer both to sovereign states and to other political entities, while other times it refers only to states. It is not uncommon for general information or statistical publications to adopt the wider definition for purposes such as illustration and comparison.

Source : Wikipedia