Many different species of flowering plants are commonly referred to as lilies. Those included in the genus Lilium, like Easter lilies, Asiatic lilies, and tiger lilies, cause severe kidney failure in cats that ingest even a small amount of the plant.
Day lilies fall into another genus (Hemerocallis) but have similar effects. All parts of the plant are toxic to cats, but the flowers are especially dangerous. In fact, just the pollen from one of these lilies has been known to be lethally poisonous to cats.