A cliche to me is an overused expression, sometimes expressing a truth, as in "You get what you pay for," or "It was raining like cats and dogs."

Spot on my own definition of cliche, Alex. Interesting, though, cos I'd say just "It was raining cats and dogs". UK difference?

I reckon a "buzzword" is usually more of a word than a phrase: "multitasking", "leveraged", "joined-up", "hot-desk", etc. Buzzwords tend to have a more limited lifespan than cliches (and good thing, too ), although some worm their way into everyday speech, and even into dictionaries.