noun
Refers to physical refuse, leftovers, or worthless remnants. Can also be used metaphorically for something of no value.
工場から出るくずをリサイクルする。
We recycle the waste from the factory.
紙くずをゴミ箱に捨てて
Please throw the paper scraps in the trash can.
noun
Strongly derogatory term for a contemptible person; used as an insult. Often written in katakana for emphasis.
あんなクズとはもう関わりたくない。
I don't want anything to do with that piece of trash anymore.
彼は人を平気で裏切る、本当のくずだ。
He betrays people without a second thought; he's real scum.
ゴミ is the standard word for everyday trash or garbage; くず emphasizes worthlessness or remnants, and is more often used metaphorically or as an insult.
カス (粕) refers to dregs or lees left after extraction, often used for physical residue; くず is broader and includes scraps, waste, and figurative trash.
The origin is uncertain; the kanji 屑 has been used historically for fragments and waste, and the reading くず is native Japanese.