In informal writing or speaking, the two words are interchangeable. But in formal writing, there is little difference between the two. "The formal rule is to use if when you have a conditional sentence and whether when you are showing that two alternatives are possible". Examples; "Roger didn't know whether John would arrive on Friday". "Roger didn't know if John would arrive on Friday". In either sentence, the meaning is that John may or may not arrive on Friday. Now examples, where the words are not interchangeable; "Roger didn't know whether John would arrive on Friday or Saturday". In addition to arriving on Friday or Saturday, there is the possibility that John may not arrive at all. These last two sentences show why it is best to use whether when you have two possibilities. Finally, use whether when you have two discrete choices or mean "regardless of whether," and use "if" for conditional sentences. Hope it helps.