BAPI vs Call Transactions

I am new to area with out any formal training other than from the job and other experienced analysts. As I have been studing and learning, questions and doubts have arosed. With everyone´s expertise, no doubt my questions will be answered in thorough and precise manner. I am in the Functional SD role of S.A.P.

What is the exact definition of BAPI and Call transaction? I am aware they call other transactions to do a certain function for example VA01 create order - VL01N to create deliveries and so on. But I would like to know the exact definition of the two.

But what is the main difference between the two?

Is call transactions becoming obsolete?

Is BAPI the way of the future for this function?

Thank you for the help..