Kale is a green leafy vegetable that's a close relative of cabbage and broccoli. Kale is tough and bitter when raw, but it can be yummy when you prepare it the right way.
If you're a fan of green smoothies, there's almost certainly some kale in them. Marinating kale, roasting it with olive oil and salt, and throwing it in a stir fry are all great ways to make kale tasty. Long ago, kale was also a slang term for "money," as in, "Hand over the kale, sweetheart, and nobody gets hurt." Kale comes from a Scottish variation on cole, which meant "cabbage" in Middle English.