Smoothies are fruit-based solid or liquid drinks whizzed up in a blender and enjoyed chilled or frozen. You can add all sorts of different ingredients to the basic fruit mix (crushed ice, milk, yoghurt, honey, herbs and more).
They're thicker than juice, more liquid than purée and less iced than sorbet, but believe it or not, smoothies have no real pre-defined conistency and there are loads of different mixtures you can come up with. It all depends on how you make them and what you put in them.