Usually there is some kind of milk or yogurt, plus ice, and usually fruit and/or juice (good ones have real fruit in them).
I am a huge smoothie fan, myself. I wish they'd put a Jamba Juice in WDW!
Also, if you're looking to try one, Auntie Gravity's has a good Strawberry/Banana smoothie. It's around $4, and is quite tasty.