a milkshake is made from blended fresh fruit and milk, and
a smootie is made from blended fresh fruit only
As it turns out though, most cafes assume the following:
So it seems, the main difference is whether flavours or fresh fruit are used. I think though that the ‘other’ ingredients are more important; i.e. whether it’s milk, ice, cream, yogurt, crushed ice, or none (in my opinion, this makes the main difference in taste) – but that’s often not so obvious. Hence, I’ve been avoiding ordering these drinks in public and prefer to make them myself the way I like them, which ends up being a lot cheaper anyway!