An expression is a general cluster of words like “don’t know which end is up.” This means to be disoriented or confused. Example: My cousin Julie is so busy, she doesn’t know which end is up.
An idiom is a colorful expression like “raining cats and dogs” (which means raining very hard). Another idiom is “doing something by the seat of your pants” (which means doing something as you go along, without prior plan). Example: This process is new for everyone on the team; we’re doing it by the seat of our pants this first time. The meanings of these idioms have nothing to do with pants or house pets, but they give us interesting ways to express ourselves.