"I am doing an academic project work. So Can you pls. explain the circuit in the basis of circuit theorems in order to attach it with project report." (my emphasis)

In other words, this is a homework assignment, and you want this forum to do your homework for you?

You can find all the information you need in the data sheet for the MAX232, and the application notes and other technical articles on the Maxim website - just use the 'Search' facility on their site.

You can also find such technical information on the other manufacturers' and other websites already mentioned in this thread.

You should be able to find the actual RS232 spec in your college library (if you can't, complain to the appropriate authorities!)

A fundamental part of "academic project work" is researching "The Literature" - ie, the body of published work already available on a subject
So; time to start practising!