Idioms, clichés, euphemisms, and colloquialisms - What are the differences and the similarities between these terms? I don't understand them.
These are all shopworn phrases of different degrees.
An idiom is a phrase with an understood meaning at odds with its literal meaning ("This joint is jumping!"). A cliche is a shopworn phrase that is no longer edgy or current ("Talk to the hand!"). A euphemism is a polite term for an unpleasant topic (like referring to a toilet as a "porcelain convenience"), and a colloquialism is an informal term one would use in private conversation, but never in an academic paper, rather the opposite of a euphemism (like referring to one's wife as a "ball and chain").