Tomatillos belong to the same family as Deadly Nightshade (Atropa belladonna).
Belladonna has been used since the middle ages as a medicine as well as a poison. To the common man, this family is also known as the rather mundane family of potatoes, capsicum (bell peppers), eggplants and tomatoes.
A tomatillos is a rather small fruit enclosed in a papery husk (calyx). It resembles an unripe tomato on the outside. Once cut, a tomatillo looks more like a gooseberry than a tomato, it is white and much meatier than a tomato.