'Difference' is where two or more things are not similar (they are dissimilar). A 'discrepancy' is where two or more things are different, but specifically when they are supposed to be similar or equal. A discrepancy is not expected, and therefore it is considered to be suprising or illogical. For example, when a suspect in a criminal investigation tells a story that disagrees with previous evidence it is a discrepancy. A 'shortfall' is where there is a deficit (or debt) in something. For example, if you need to pay $10 but you only have $5, you have a shortfall of $5. Shortfalls are also usually unexpected, like a 'discrepancy.' Hope this helps. :)