If is a conditional; whether introduces an alternative. Tell me if you’ll come means you should only respond if you are coming; Tell me whether you’ll come asks for a response no matter your decision. Be aware that the conditional if can be better hidden than in the previous example: He asks if he may go home technically means he asks a question once given the permission to go home; He asks whether he may go home means he asks for permission to go home. In informal situations, however, if and whether function more or less as equals.