A comparison of apples and oranges occurs when two items or groups of items are compared that cannot be practically compared, typically because of inherent or fundamental differences between the objects.
The idiom, comparing apples and oranges, refers to the differences between items which are popularly thought to be incomparable or incommensurable, such as apples and oranges. The idiom may also indicate that a false analogy has been made between two items, such as where an apple is faulted for not being a good orange.