A different kettle of fish
Definition 1 of 1
Pro Tip 1/3Idiomatic Nuance
This phrase implies contrast, usually more than just a simple difference, and often suggests a situation is unique or unusual. 
If you can drive a car, riding a motorcycle will be a different kettle of fish.
Pro Tip 2/3Cultural Origin
This idiom is of British origin, so it might be more recognized and understood in UK English than in other varieties of English. 
Backpacking through Europe is quite a different kettle of fish than a guided tour.
Pro Tip 3/3Avoid Literality
The phrase doesn't relate to fish or kettles; understand it's a figurative expression for saying something is different. 
Trading stocks is a different kettle of fish from saving money in a bank.