Are tomatillos and green tomatoes the same?

They look the same and come from the same nightshade family of tomatoes. Green tomatoes are an unripe version of the mature tomato but tomatillos look like green tomatoes, even when they're ripe.

The most obvious difference is the papery husk but, even when you cut into the fruit, you will see the difference.

I wouldn't substitute green tomatoes for tomatillos because green tomatoes could cause upset stomach. You could substitute mature tomatoes for tomatillos, though. It would give a different taste and color, but I'm sure it would still be tasty.