What is the difference between set phrase and catch phrase?
Do the expressions describe something without relation with each other?
set phrase an unvarying phrase having a specific meaning, such as “raining cats and dogs,” or being the only context in which a word appears, e.g., “aback” in “take aback.”
catch phrase A phrase in wide or popular use, especially one serving as a slogan for a group or movement.
The first is widely used and a synonym for cliché. The second is usually more specifically applied. "It's morning in America" was the catch phrase used to get Ronald Reagan elected president of the U.S. But it had no wider currency than that.