Tomatillos are a nightshade native to Mexico and were well developed as a crop by the Aztecs. Our first evidence of cultivation is from around 800 BCE but it was probably grown far earlier than that.
Tomatillos are harvested when medium to light green and used at that stage of ripeness. The green skined ones are commonly sold in Southern California, but purple skinned ones are sometimes available. The inside is green and white for all of them.
Tomatillos are much used in Mexican green sauces and salsas. They are both tart and sweet. There is no satisfactory substitute. The small Milpero variety is a little stronger in flavor and a little less tart. For most uses I prefer the large ones.
The regular large ones typically around 2-1/4 inches diameter and 2-3/4 ounces, The largest I've seen was 3-1/3 inches diameter and 6-5/8 ounces. The small Milpero variety is typically around 1-1/4 inches diameter and weighs 1/2 ounce.
More on Physalis - Husk Tomatoes.
Tomatillos are pleasant to eat both raw and cooked. They are used both ways to make green sauces (salsa verde), for which they are an essential and irreplaceable ingredient. Fortunately they are now so common in the U.S. lack of a substitute is no longer a problem.