Use it when items or concepts don't share enough similarities for a fair comparison. I can't decide if I prefer music or painting; it's apples and oranges.
Pro Tip 2/3
Informal Context
This phrase is casual and usually not used in formal analysis or academic writing. Talking about soccer and baseball, my friend said they're apples and oranges.
Pro Tip 3/3
Avoid Overuse
Be wary of using it too often; it's best for situations with clear and strong contrasts. Saying film and digital photography are apples and oranges might not always hold true.