Dear Mr. Debashish, let me try if I can though it is little bit difficult to make one understand on line .
classical songs are the songs which are based on a particual raga ..... like jhanak jhanka todi baje payeliya sung by Manna Dey is purely compsed in Rag Darbari-Kanada set to teen taal or trital. another song of Md. Rafi ... Madhuvan me radhika nache re from Kohinur is absolutely In Rag Hamir .
Here ,the relevant question is what is rag? A rag is a melodious composition of notes . every rag has to obey certain principle ...... these are quite technical terms
The moment it breeches the princple of the said raga ... we cant term that as rag . That will be termed as Mishra , in our film industry most of the compositions are like that .
One has to learn and study ragas in order to identify it . some times it is same to hear for the lay man but technically knowledgable person only can easily find out that .
If you are not satisfied with the answer ,please write to