The answer is dependent on the area of study (e.g., nation).  Ethnic groups vary across nations, and not all ethnic groups appear in nor are necessarily relevant to all sociological contexts.  Sociologically, ethnic groups have shared histories, and those histories are a produce of sociological produced "inheritance" -- shared experiences.

Thank you very much, sir, for your response.

Actually in the Indian context, we have religion and caste (social groups) but in the context of  American, African and Europian countries I found the social division on the basis of ethnic groups.  So for the beginner researcher like me, who want to study cross-country comparison of the population  find very difficult to understand the actual meaning of ethnic groups in social sciences research. 

I also found that it vary from region to region and have no fixed defining characteristics. 

I would have to concur with both of my previous colleagues the sociological perspective varies dependant on the country it's ethnic make up where all its people's are and the ins visual beliefs that they have and how they Carr through the daily way of life and m no with and to each other. If you look at some of my contributions I may have uploaded an article related to sociologhy and ethnicity check out what I have added and see if the article I completed would help.