What is the origin of the idiom "a different kettle of fish" and when to use it?

The idiom "a different kettle of fish" is believed to have originated in the 18th century. It is thought to have come from the custom of preparing fish in a large pot, or kettle. If the fish were different types of fish, the cooking process would be different for each type. This led to this phrase being used to describe any situation that was different from another.