A country is an area of land with boundaries describing a defined self governing population. There are many countries throughout the world made from different races and cultures. The idea of creating borders has been one of the prime reasons for wars and peace throughout our history. By setting defined lines around a land area the population inside are effectively setting their safety zone. You cross this border and you better be ready for trouble. Of course in the modern world these borders are important for many different reasons.
Governments need to know who is inside their borders for welfare and policing. They need to know who visits and who wishes to emigrate. The borders lets a government know if a person should be paying tax in their country or not.
A country is really just a group thinking for the people inside. This itself has led to sections of countries breaking away. Take Pakistan for instance. Originally part of India but a large population of Muslims who felt that they did not have equal standing in a nation with Hindu majority wanted their own independence. Eventually Pakistan was formed with outside help and a new country was born.
Author: Jonathan Sayles, my google+ page