How national anthem is different from national song?

The word ‘anthem’ has come to be associated with any song or hymn of praise, joy, triumph, pride etc.

It is a song specifically identified with a particular group, body, cause, country, organization etc. National anthem is the most familiar use of the word anthem.  

National anthem is a patriotic hymn or song and is officially adopted by a country as an expression of national identity. It reflects the culture, tradition and history of that country. It is compulsorily sung in the governmental and other official ceremonies.  Usually the lyrics of the national anthem are in the country’s official language. However Indian national anthem ‘Jana Gana Mana’ is in ‘sanskritised Bengali’ language. 

A national song is also extremely popular like the national anthem. However it may not have official recognition and not as often sung in a ceremonial function as national anthem. While there are regulations governing the mode of singing of a national anthem, usually there are no specific rules governing singing of national song. ‘Vande Mataram’ is widely recognized as national song of India.