There are recipes for green tomatoes that use just that - green (unripe) tomatoes. I make relish when I have enough green tomatoes in the fall (picked before the first frost). Tomatillos are similar, but they have that husk (calyx) and aren't in the same genus, though they're all Solanaceae family. They're apparently more tart, though when they're quite ripe they're sweeter and similar to the tomato and can be substituted.